rtc: at91rm9200: stop calculating yday in at91_rtc_readalarm

Calculating yday in the read_alarm callback is useless as this value is
never used later. Also, it was buggy anyway because at the time this is
done, tm_year is always 0 as the alarm register doesn't hold the year.

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>