rtc: rtc-nuc900: use NULL instead of 0

check_rtc_access_enable() returns pointer, thus NULL should be used instead of 0 in order to fix the following sparse warning:

drivers/rtc/rtc-nuc900.c:102:16: warning: Using plain integer as NULL pointer

---
 drivers/rtc/rtc-nuc900.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 22861c5..248653c 100644
@@ -99,7 +99,7 @@ static int *check_rtc_access_enable(struct nuc900_rtc *nuc900_rtc)
 	if (!timeout)
 		return ERR_PTR(-EPERM);
 
-	return 0;
+	return NULL;
 }
 
 static int nuc900_rtc_bcd2bin(unsigned int timereg,

1.7.10.4