sh: Turn off IRQs around get_timer_offset() calls.

Since all of the sys_timer sources currently do this on their own
within the ->get_offset() path, it's more sensible to just have
the caller take care of it when grabbing xtime_lock. Incidentally,
this is more in line with what others (ie, ARM) are doing already.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>