rtc: rtc-s3c: fix RTC initialization method

Change RTC initialization method in probe().  The 'rtc_valid_tm(tm)' can
check whether RTC BCD is valid or not.  And change the method of checking
because the previous method cannot validate RTC BCD registers properly.

Signed-off-by: Changhwan Youn <chaos.youn@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Cc: Wan ZongShun <mcuos.com@gmail.com>
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>