[media] cec: replace cec_report_features by cec_fill_msg_report_features

The fill function just fills in the cec_msg struct, it doesn't transmit
the message. This is now done explicitly.

This makes it possible to switch to transmitting this message with adap->lock
held.

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