media: cec-api: log the reason for the -EINVAL in cec_s_mode

If cec_debug >= 1 then log why the requested mode returned -EINVAL.

It can be hard to debug this since -EINVAL can be returned for many
reasons. So this should help.

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