greybus: es{1,2}: remove control endpoint field

There is no need to store the endpoint number of the control requests since
the default control endpoint is used and the USB standard defines for it a fixed
endpoint number of 0.

Remove every instance of the field control_endpoint and replace it with a
hardcoded 0 value.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>