media: cros-ec-cec: Get number of CEC ports from EC

Add a new CEC port count host command and use it to query the number of
CEC ports from the EC. If the host command is not supported then it must
be old EC firmware which only supports one port, so fall back to
assuming one port.

This patch completes support for multiple ports in cros-ec-cec.

Signed-off-by: Reka Norman <rekanorman@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>