rtc-pm8xxx: Fix issue in RTC write path

In order to set time in rtc, need to disable
rtc hw before writing into rtc registers.

Also fixes disabling of alarm while setting
rtc time.

Change-Id: I0ffad7b6dcba5fea510d5acfad82ed328cffcf84
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>