[media] cec: clear all status fields before transmit and always fill in sequence

Before transmitting a message clear all status fields and always fill
in the sequence number. Make sure the sequence number is never 0.

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