greybus: es1-ap-usb: handle -EPROTO in check_urb_status()

On a disconnect we can also have a status of -EPROTO. This results in
a flood of error messages due to the -EAGAIN handling of unsupported
status results. Fix this by also returning status when we have -EPROTO.

Signed-off-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>