rtc: fix potential race condition

RTC drivers must not return an error after device registration.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Reported-by: Ales Novak <alnovak@suse.cz>
Cc: Alexander Shiyan <shc_work@mail.ru>
Cc: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Srikanth Srinivasan <srikanth.srinivasan@freescale.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>