[media] rc: unlock on error in show_protocols()

We recently introduced a new return -ENODEV in this function but we need
to unlock before returning.

[mchehab@redhat.com: found two patches with the same fix. Merged SOB's/acks into one patch]
Acked-by: Herton R. Krzesinski <herton.krzesinski@canonical.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable@vger.kernel.org
Signed-off-by: Douglas Bagnall <douglas@paradise.net.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>