power: pm8921-bms: use mutex even for debug entries

Reading of the soc parameters should always be done after grabbing
the mutex. There are some debugfs entries that do not do so.

Fix them.

Change-Id: I77b40f9cb8295cff1d4a24dfc7d85b0e57262f6e
CRs-Fixed: 390074
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>