[MIPS] Sibyte: Stop timers before programming next even.

We have no guarantee by the generic time code that the timer is stopped
when the ->next_event method is called.  Modifying the Timer Initial Count
register while the timer is enabled has UNPREDICTABLE effect according to
the BCM1250/BCM1125/BCM1125H User Manual.  So stop the timer before
reprogramming.

This is a paranoia fix; no ill effects have been observed previously.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>