PM / devfreq: bw_hwmon: Fix event-based race condition

Two events caused by sysfs writes to the bw_hwmon governor, when properly
interleaved, can lead to a race condition due to needing to stop and
restart the hardware monitor.  Fix this by adding locking to the event
handler.

Change-Id: I85c120ac0cf53f59ec12b8f56aed494acc446258
Signed-off-by: Jonathan Avila <avilaj@codeaurora.org>