rtc: ds1511: let the core know when alarm are not supported

Instead of failing function calls, let the core know alarms are not
supported so it can fail early and avoid unnecessary calls.

Link: https://lore.kernel.org/r/20240227230431.1837717-8-alexandre.belloni@bootlin.com


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