greybus: audio: mark the data connection as offloaded

Mark the data-connection as offloaded, that is, under control of the
host device (AP-bridge).

This prevents messages from being sent from or forwarded to the AP.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>