usb: dwc3: gadget: don't send extra ZLP

If the request->length is zero, a ZLP should already be sent due to that
and another ZLP is not needed to terminate the transfer.

Fixes: 04c03d10 ("usb: dwc3: gadget: handle request->zero")
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>