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