greybus: interface: send deactivate prepare when interface is disabled

The AP Interface shall exchange a Greybus Control Interface Deactivate
Prepare Operation with the Interface being powered down.

Testing Done:
 - Check for the return code after sending the deactivate prepare operation

Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Alex Elder <elder@linaro.org>