media: cec: move check from cec_transmit to cec_transmit_msg_fh

This ensures all the cec_msg checks are done in the same place.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>