rtc: rtc-s3c: add rtc_valid_tm in s3c_rtc_gettime()

Add "rtc_valid_tm" in s3c_rtc_gettime() as per Wan ZongShun's suggestion.

Suggested-by: Wan ZongShun <mcuos.com@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>