power: vm-bms: Add coarse level mutex lock in monitor_soc_work

There is a race between lock/unlock of monitor_soc_work
and get_prop_capacity. Fix this by executing the complete
monitor_soc_work and get_prop_capacity with the lock held.

CRs-Fixed: 648888
Change-Id: I276c2510eddab58ff6aa3ea856f1d5688a751679
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>