drivers/rtc/rtc-nuc900.c: 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

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>