drivers/rtc/rtc-mc13xxx.c: fix potential race condition

RTC drivers must not return an error after device registration.  This
patch makes RTC registration as the last action.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>