rtc: stop warning for invalid alarms when the alarm is disabled

When the alarm is not enabled, it may never have been set and so we can't
expect it to be valid. This will prevent the apparition of boot messages
like this one:

rtc rtc0: invalid alarm value: 2023-7-8 45:85:85

Link: https://lore.kernel.org/r/20230827221532.543353-1-alexandre.belloni@bootlin.com


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