drivers: rtc: Wrong alarm mask in rtc_alarm_get_time

Fixing 3 issues:

1. The mask can be wrong if the alarm register is
set to a MAX value because the alarm bit is the
highest in the alarm register. The mask is now
generated by checking the AE_x bits in the time
registers.

2. Fixing possible NULL pointer exception in
alarm_set_time API. timeptr can be set to NULL
with mask 0 in the alarm_set_time function.
The regs variable for the I2C communication
is written with the correct value from timeptr
only when the right bit in the mask is set.

3. rv8263c8_alarm_set_time() now resets the
alarm status.

4. Interrupts are now enabled by using
rv8263c8_alarm_set_time() rather than when
setting an alarm callback.

Signed-off-by: Daniel Kampert <DanielKampert@kampis-elektroecke.de>