[media] cec: add check if adapter is unregistered

CEC USB dongles can be unplugged at any time, and at that point they will
be unregistered. Make sure that any attempt afterwards to set the physical
or logical addresses will be ignored.

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