[media] cx88: protect cx8802_devlist with a mutex

Add and use a mutex to protect the cx88-mpeg device list.  Previously
the BKL prevented races.

Based on work by Ben Hutchings <ben@decadent.org.uk>.

Tested-by: Andi Huber <hobrom@gmx.at>
Tested-by: Marlon de Boer <marlon@hyves.nl>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>