Don't skip empty transfers. In isochronous transfers, make sure (#72)

Don't skip empty transfers.
Update the fake libusb to honor the endpoint max packet sizes.
Update tests that were taking advantage of the fact that libusb
allowed unlimited amount of data in any transfer packet.

endpoint.go

@@ -86,10 +86,6 @@ func (e *endpoint) String() string {
 }
 
 func (e *endpoint) transfer(ctx context.Context, buf []byte) (int, error) {
-	if len(buf) == 0 {
-		return 0, nil
-	}
-
 	t, err := newUSBTransfer(e.ctx, e.h, &e.Desc, len(buf))
 	if err != nil {
 		return 0, err