Improve tests - use offsetof to find the position of the iso packet

descriptor in the transfer struct.

usb/endpoint.go

@@ -62,7 +62,7 @@ func (e *endpoint) transfer(buf []byte, timeout time.Duration) (int, error) {
 	}
 
 	tt := e.TransferType()
-	t, err := newUSBTransfer(e.Device.handle, e.EndpointInfo, buf, timeout)
+	t, err := newUSBTransfer((*deviceHandle)(e.Device.handle), e.EndpointInfo, buf, timeout)
 	if err != nil {
 		return 0, err
 	}