[media] cec: zero counters in cec_received_msg()

Make sure the TX counters are zeroed in the cec_msg struct.
Non-zero TX counters make no sense when a message is received,
and applications should not see non-zero values here.

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