drivers/rtc/rtc-twl.c: optimize IRQ bit access

As the TWL RTC driver has a cached copy of enabled RTC interrupt bits in
variable rtc_irq_bits, that can be checked before really setting or
masking any of the interrupt bits.

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>