[media] drivers/media/media-devnode: clear private_data before put_device()

Callbacks invoked from put_device() may free the struct media_devnode
pointer, so any cleanup needs to be done before put_device().

Signed-off-by: Max Kellermann <max@duempel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>