rtc: cmos: Reject unsupported alarm values

Some platforms allows to specify the month and day of the month in
which an alarm should go off, some others the day of the month and
some others just the time.

Currently any given value is accepted by the driver and only the
supported fields are used to program the hardware. As consequence,
alarms are potentially programmed to go off in the wrong moment.

Fix this by rejecting any unsupported value.

Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>