[media] em28xx-v4l: move v4l2_ctrl_handler freeing and v4l2_device unregistration to em28xx_v4l2_fini

v4l2_ctrl_handler_free() and v4l2_device_unregister() are currently only called
when the last user closes the device and the device is already disconnected.
But that's wrong, we need to call these functions whenever the em28xx-v4l
extension is closed and we can already do this if the device is still opened
by some users.
Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>