[media] cec: zero unused msg part after msg->len

Ensure that the unused part of the msg array is zeroed. This is
required by CEC 2.0 when receiving shorter messages than expected.
In that case the remaining bytes are assumed to be 0.

There are no such CEC messages yet, but it is required to be future proof.

And since we're doing it for received messages, do it for transmitted
messages as well. It's unambiguous this way.

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