media: cec: add adap_free op

This is needed for CEC adapters that allocate resources that have
to be freed before the cec_adapter is deleted.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>