media: media/cec: use CEC_MAX_MSG_SIZE instead of hardcoded 16

Use the proper define for the maximum CEC message length instead of
hardcoding it.

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