[COMMANDS] Fix the month number handling in wallclock settime

Wallclock settime accepts date/time string like:
wallclock set_time 06:45:00 22 05 2013
but this command string sets the month to the next month.
Problem is make time command is called by month + 1. This
is okay when month is string jan, feb etc because month
number in code will start from 0, 1 etc. Human will enter
month 1-12 so fix is to decrease the value before giving
to mktime.

Signed-off-by: Himanshu Chauhan <hschauhan@nulltrace.org>