media: Clear devnode->media_dev holding media_devnode_lock

Fix media_devnode_unregister() to clear devnode->media_dev while holding
media_devnode_lock. media_devnode_register()'s cdev_add_error handling
does this correctly.

Signed-off-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>