media: cros-ec-cec: limit msg.len to CEC_MAX_MSG_SIZE

[ Upstream commit 2dc73b48 ]

I expect that the hardware will have limited this to 16, but just in
case it hasn't, check for this corner case.

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