greybus: hd: add cport_shutdown callback

Add a host-device cport_shutdown callback which will be called as part
of the new connection tear-down sequence for offloaded connection in
order to do a cport_shutdown operation on behalf of the AP.

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