rtc: rtc-s3c: fix setting missing field of getalarm

Current s3c_rtc_getalarm() sets missing field of alarm time with 0xff.
But this value should be -1 according to drivers/rtc/interface.c.

Signed-off-by: Changhwan Youn <chaos.youn@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Acked-by: 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>