drivers/rtc/rtc-max77686.c: use devm_kzalloc()

Use devm_kzalloc() to make cleanup paths more simple.

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>