usb: Add USB_RET_ADD_TO_QUEUE packet result code This can be used by usb-device code which wishes to process an entire endpoint queue at once, to do this the usb-device code returns USB_RET_ADD_TO_QUEUE from its handle_data class method and defines a flush_ep_queue class method to call when the hcd is done queuing up packets. Signed-off-by: Hans de Goede <hdegoede@redhat.com> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>