rtc/mc13783: fix use after free bug

This was introduced by v2.6.34-rc1~38:

	4c014e87

 (rtc/mc13783: protect rtc {,un}registration by mc13783 lock)

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>