[media] v4l2-ctrls: Correctly destroy mutex in v4l2_ctrl_handler_free()

The mutex that was initialised in v4l2_ctrl_handler_init_class() was not
destroyed in v4l2_ctrl_handler_free(). Do that.

Additionally, explicitly refer to the ctrl handler's mutex in mutex
initialisation for clarity.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>