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: I0af7d8b3025d036cfa8927c2e37ef865075ff312
Signed-off-by: Pan Fang <fangpan@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
Signed-off-by: Chet Kener <Cl3Kener@gmail.com>