media: dvb: mpq: Removed unnecessary mutex locks

Removed unnecessary mutex lock in the function mpq_get_dev_frm_client.
The global structure is written only at the time of open and close and
hence no mutex protection is required for reading.

Change-Id: I9b5190bfbdf952d35935f5fbd7157d9e3d23ed3a
Signed-off-by: Bamidi RaviKiran <bamidir@codeaurora.org>