usb: dwc3: ep0: fix DMA starvation by assigning req->trb on ep0

usb: dwc3: ep0: fix DMA starvation by assigning req->trb on ep0

commit 55168470835688e5da5828cdcf1b1498d7baadb1 upstream.

If we don't assign a TRB to ep0 requests, we won't be able to unmap
the request later on resulting in starvation of DMA resources.

Fixes: 4a71fcb8ac5f ("usb: dwc3: gadget: only unmap requests from DMA if mapped")
Reported-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Tested-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>