RTC: sa1100: remove redundant code of setting alarm

In rtc generic interface of setting alarm, it will check the alarm time
value valid or not, so rtc_periodic_alarm is redundant, remove it.

Signed-off-by: Jett.Zhou <jtzhou@marvell.com>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>