drivers/rtc/rtc-max77686.c: Fix wrong register

Fix a read of the wrong register when checking whether the RTC timer has
reached the alarm time.

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>
Signed-off-by: Myugnjoo Ham <myungjoo.ham@samsung.com>
Reviewed-by: Jonghwa Lee <jonghwa3.lee@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>