rtc: rv8803: fix off-by-one in month counting

tm_mon has a range from 0..11, but the RTC expects 1..12. So we adapt
the month accordingly. This was determined when comparing the driver
with the corresponding linux kernel driver.

Signed-off-by: Oliver Graute <oliver.graute@kococonnector.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Reviewed-by: Heiko Schocher <hs@denx.de>