media: v4l2-subdev: handle module refcounting here
The module ownership refcounting was done in media_entity_get/put,
but that was very confusing and it did not work either in case an
application had a v4l-subdevX device open and the module was
unbound. When the v4l-subdevX device was closed the media_entity_put
was never called and the module refcount was left one too high, making
it impossible to unload it.
Since v4l2-subdev.c was the only place where media_entity_get/put was
called, just move the functionality to v4l2-subdev.c and drop those
confusing entity functions.
Store the module in subdev_fh so module_put no longer depends on
the media_entity struct.