[media] cec: ignore messages that we initiated

Some CEC adapters will receive messages that they initiated. Add a
check that will ignore such messages.

Most hardware behaves correctly in this respect, but I have seen
adapters that don't, so just filter this out in the framework.

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