media: cros-ec-cec: Support multiple ports in set/get host commands

Reuse the top four bits of the cmd field to specify the port number.
The reason for doing this as opposed to adding a separate uint8_t field
is it avoids the need to add new versions of these commands. The change
is backwards compatible since these bits were previously always zero, so
the default behaviour is to always operate on port 0.

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