[media] cec: fix report_current_latency

In the (very) small print of the REPORT_CURRENT_LATENCY message there is a
line that says that the last byte of the message (audio out delay) is only
present if the 'audio out compensated' value is 3.

I missed this, and so if this message was sent with a total length of 6 (i.e.
without the audio out delay byte), then it was rejected by the framework
since a minimum length of 7 was expected.

Fix this minimum length check and update the wrappers in cec-funcs.h to do
the right thing based on the message length.

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