PM/devfreq: bw_hwmon: add mutex lock in polling interval case

Protect polling interval update event in the governor
with mutex lock.
This will avoid parallel execution of update interval event
on different cores, resulting in memory poison overwritten.

CRs-Fixed: 2035633
Change-Id: Ib919d9774317bc01e3cf581c169c5d18555f484f
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>