rtc: rtc-sh: fix rtc for out-by-one for the month.

The RMONCNT register, which holds the month in the RTC, takes a value
between 1 and 12 while the tm_mon field in the time structures takes
a value between 0 and 11. This wasn't being taken into account in
rtc-sh resulting in the month being out by one.

eg, on my board during boot the RTC is set to:

  RTC is set to Thu Jul 01 09:00:00 1999

but "hwclock -r" immediately after logging in was showing:

  Sun Aug  1 09:01:43 1999  0.000000 seconds

Signed-off-by: Jamie Lenehan <lenehan@twibble.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>