rtc: at91rm9200: avoid time readout in at91_rtc_setalarm

The current rtc time is read out in at91_rtc_setalarm but it it immediately
overwritten by the alarm time. Remove the readout.

Link: https://lore.kernel.org/r/20191229204421.337612-5-alexandre.belloni@bootlin.com


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