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

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>

drivers/usb/dwc3/ep0.c

@@ -990,6 +990,8 @@ static void __dwc3_ep0_do_control_data(struct dwc3 *dwc,
 					 DWC3_TRBCTL_CONTROL_DATA,
 					 true);
 
+		req->trb = &dwc->ep0_trb[dep->trb_enqueue - 1];
+
 		/* Now prepare one extra TRB to align transfer size */
 		dwc3_ep0_prepare_one_trb(dep, dwc->bounce_addr,
 					 maxpacket - rem,
@@ -1015,6 +1017,8 @@ static void __dwc3_ep0_do_control_data(struct dwc3 *dwc,
 					 DWC3_TRBCTL_CONTROL_DATA,
 					 true);
 
+		req->trb = &dwc->ep0_trb[dep->trb_enqueue - 1];
+
 		/* Now prepare one extra TRB to align transfer size */
 		dwc3_ep0_prepare_one_trb(dep, dwc->bounce_addr,
 					 0, DWC3_TRBCTL_CONTROL_DATA,
@@ -1029,6 +1033,9 @@ static void __dwc3_ep0_do_control_data(struct dwc3 *dwc,
 		dwc3_ep0_prepare_one_trb(dep, req->request.dma,
 				req->request.length, DWC3_TRBCTL_CONTROL_DATA,
 				false);
+
+		req->trb = &dwc->ep0_trb[dep->trb_enqueue];
+
 		ret = dwc3_ep0_start_trans(dep);
 	}