[media] cec: document the error codes

Document all the various error codes returned by the CEC ioctls.

These were never documented, instead the documentation relied on a reference
to the generic error codes, but that's not sufficient.

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