drivers/rtc/rtc-twl.c: return correct RTC event from ISR

Following changes are made as part of this change:

1. As TWL RTC supports periodic interrupt, the correct event should be
   RTC_PF instead of RTC_UF.

2. No need to initialize variable "events" to 0 & then OR it with the
   event values.  Hence fixing it.

Signed-off-by: Venu Byravarasu <vbyravarasu@nvidia.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>