horus3a: fix compiler warning

Shut up this compiler warning that I get during the daily build:

horus3a.c: In function 'horus3a_set_params':
horus3a.c:308:24: warning: 'rolloff' may be used uninitialized in this function [-Wmaybe-uninitialized]

     symbol_rate * (100 + rolloff), 200000) + 5;
                        ^

index 5074305..000606a 100644
@@ -285,6 +285,7 @@ static int horus3a_set_params(struct dvb_frontend *fe)
 			rolloff = 20;
 			break;
 		case ROLLOFF_AUTO:
+		default:
 			dev_err(&priv->i2c->dev,
 				"horus3a: auto roll-off is not supported\n");
 			return -EINVAL;