[media] drivers/media/media-device: move debug log before _devnode_unregister()

After media_devnode_unregister(), the struct media_device may be freed
already, and dereferencing it may crash.

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