ds1511_rtc_update_alarm is called twice but one of the call is overkill as
it only has to enable or disable the alarm instead of updating all the alarm
registers. Merge it in its main call site and introduce a new finction to
enable or disable the alarm.

Link: https://lore.kernel.org/r/20240227230431.1837717-7-alexandre.belloni@bootlin.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>