driver/rtc/class.c: check the error after rtc_read_time()

In rtc_suspend() and rtc_resume(), 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>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>