rtc: pm8xxx: clear alarm register when alarm is not enabled

Clear alarm register when alarm is not enabled otherwise the consumer
may still start alarm timer if it find the alarm register is not zero.

Signed-off-by: hankecai <hankecai@vivo.com>
Link: https://lore.kernel.org/r/APoAZgAaCEiRpKG6PlzreaqE.1.1584791417367.Hmail.hankecai@vivo.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>