usb: dwc3: gadget: Handle ZLP for sg requests

[ Upstream commit bc9a2e226ea95e1699f7590845554de095308b75 ]

Currently dwc3 doesn't handle usb_request->zero for SG requests. This
change checks and prepares extra TRBs for the ZLP for SG requests.

Cc: <stable@vger.kernel.org> # v4.5+
Fixes: 04c03d10 ("usb: dwc3: gadget: handle request->zero")
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>