drivers/rtc/interface.c: check the error after __rtc_read_time()

In __rtc_set_alarm(), the error after __rtc_read_time() is not checked.
If rtc device fail to read time, we cannot guarantee the following
process.

Add the verification code for returned __rtc_read_time() error.

Signed-off-by: Hyogi Gim <hyogi.gim@lge.com>
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>