staging:iio:ad7291 fix ad7291_threshold_reg()

ad7291_threshold_reg() should return the register number of a threshold register, and thus be safe without handling a default case.

---

index f5d29ef..b868c64 100644
@@ -174,8 +174,6 @@ static unsigned int ad7291_threshold_reg(const struct iio_chan_spec *chan,
 	case IIO_TEMP:
 		offset = 8;
 		break;
-	default:
-	    return 0;
 	}
 
 	switch (info) {

@@ -186,10 +184,7 @@ static unsigned int ad7291_threshold_reg(const struct iio_chan_spec *chan,
 			return AD7291_DATA_LOW(offset);
 	case IIO_EV_INFO_HYSTERESIS:
 		return AD7291_HYST(offset);
-	default:
-		break;
 	}
-	return 0;
 }
 
 static int ad7291_read_event_value(struct iio_dev *indio_dev,

Whilst the code is bug free without those defaults currently I'd rather keep them there to suppress the warnings we will get without them...

Also without an explanation, please don't stick 'fix' in the title - makes my fingers want to send it to stable.

Warning wise...
e.g. with gcc4.9 on x86_64 I get the following tiny output ;)

CC [M] drivers/staging/iio/adc/ad7291.o
drivers/staging/iio/adc/ad7291.c: In function ‘ad7291_threshold_reg’:
drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_CURRENT’ not handled in switch [-Wswitch]
switch (chan->type) {
^
drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_POWER’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_ACCEL’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_ANGL_VEL’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_MAGN’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_LIGHT’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_INTENSITY’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_PROXIMITY’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_INCLI’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_ROT’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_ANGL’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_TIMESTAMP’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_CAPACITANCE’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_ALTVOLTAGE’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_CCT’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_PRESSURE’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:170:2: warning: enumeration value ‘IIO_HUMIDITYRELATIVE’ not handled in switch [-Wswitch] drivers/staging/iio/adc/ad7291.c:179:2: warning: enumeration value ‘IIO_EV_INFO_ENABLE’ not handled in switch [-Wswitch]
switch (info) {
^
drivers/staging/iio/adc/ad7291.c:188:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
drivers/staging/iio/adc/ad7291.c: In function ‘ad7291_threshold_reg.isra.5’:
drivers/staging/iio/adc/ad7291.c:45:31: warning: ‘offset’ may be used uninitialized in this function [-Wmaybe-uninitialized]

 #define AD7291_HYST(x)   ((x) * 3 + 0x6)
                               ^
drivers/staging/iio/adc/ad7291.c:168:15: note: ‘offset’ was declared here
  unsigned int offset;
               ^