msm:rpm-log: add the mutex to protect read opertion

This commit will protect rpm log read operation. Then multithread
read the same rpm log buffer will not corruption.

CRs-Fixed: 646485
Change-Id: I38b10c89902a9268fc0a4d649267d3a3df9a739a
Signed-off-by: Pan Fang <fangpan@codeaurora.org>