usb: dwc3: gadget: Improve TRB ZLP setup

For OUT requests that requires extra TRBs for ZLP. We don't need to
prepare the 0-length TRB and simply prepare the MPS size TRB. This
reduces 1 TRB needed to prepare for ZLP.

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>