rtc: as3722: correct month value

The RTC month value is 1-indexed, but the kernel assumes it is 0-indexed.
This may result in the RTC not rolling over correctly.

Signed-off-by: Bibek Basu <bbasu@nvidia.com>
Signed-off-by: Felix Janda <felix.janda@posteo.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>