rtc: rtc-sh: fix for period rtc interrupts.

When testing the per second interrupt support (RTC_UIE_ON/RTC_UIE_OFF)
of the new RTC system it would die in sh_rtc_interrupt due to a null
ptr dereference. The following gets it working correctly.

Signed-off-by: Jamie Lenehan <lenehan@twibble.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>