drivers/rtc/rtc-twl.c: simplify RTC interrupt clearing

For clearing RTC interrupt, programming ALARM bit only is sufficient, as
all other bits are any way not affected by writing 0 to them.

Hence removed unwanted OR operation.

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>