[media] cx25821: remove unnecessary global devlist

This device list is not necessary. The kernel already has all that information,
so just use that instead.
Also remove a bogus refcount and some dead 'private_free' code in the alsa driver.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>