rtc: isl12057: let the rtc core interpret the partial alarm

The rtc chip doesn't support triggering on month and year. So just don't
assign the respective fields in .read_alarm and let the rtc core
interpret this accordingly.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>