rtc-bfin: shrink/optimize interrupt handler a bit

By unifying the RTC_ISTAT clearing steps, we shrink the interrupt handler
and avoid multiple writes to the hardware registers.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>