[media] m5mols: Protect driver data with a mutex

Without the locking the driver's data could get corrupted when the subdev
is accessed from user space and from host driver by multiple processes.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>