[media] radio-mr800: fix locking order

Don't hold the lock before unregistering the device, since when the
device is unregistered the datastruct containing the lock may be freed
(if the refcount went to 0).

Also fixed the framework documentation that erroneously suggested the
wrong locking order as well.

Reported-by: David Ellingsworth <david@identd.dyndns.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Acked-by: David Ellingsworth <david@identd.dyndns.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>