media-devnode: add missing mutex lock in error handler

commit 88336e17

 upstream.

We should protect the device unregister patch too, at the error
condition.

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