soc: msm-pcm: Add mutex lock to protect prvt data

Add mutex lock to protect private data in _put() and
get() calls.

Change-Id: I92f5a6515b6d1c4ad650a7dcf22a0a231a84dd30
Signed-off-by: Prasad Kumpatla <nkumpat@codeaurora.org>
Signed-off-by: Soumya Managoli <smanag@codeaurora.org>