rtc: at91sam9: Remove unnecessary offset variable checks

The offset variable is checked by at91_rtc_readalarm(), but this check
is unnecessary because the previous check knew that the value of this
variable was not 0.
This removes that unnecessary offset variable checks.

Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210708051340.341345-1-nobuhiro1.iwamatsu@toshiba.co.jp