[media] cec: sanitize msg.flags

The CEC_MSG_FL_REPLY_TO_FOLLOWERS message flag only makes sense for transmitted
messages where you want to wait for the reply.

Clear the flag in all other cases.

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