rtc: max8997: Fix the returned value in case of error in 'max8997_rtc_read_alarm()'

In case of error, we return 0.
This is spurious and not consistent with the other functions of the driver.
Propagate the error code instead.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>