rtc: pcf2127: don't add/subtract 1 to tm_mon

As noted in rtc_def.h, the tm_mon field in struct rtc_time is 1-12,
unlike in struct tm where it is 0-11. Currently, running "date" prints
the wrong

Date: 2020-04-01 (Friday)    Time: 13:05:30

and setting the RTC via the date command is also broken.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>