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: 4a71fcb8 ("usb: dwc3: gadget: only unmap requests from DMA if mapped")
Reported-by: NThinh Nguyen <Thinh.Nguyen@synopsys.com>
Tested-by: NThinh Nguyen <Thinh.Nguyen@synopsys.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>