[media] cec-funcs.h: add missing const modifier

The cec_ops_* functions never touch cec_msg, so mark it as const.

This was done for some of the cec_ops_ functions, but not all.

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