usb: dwc3: gadget: prevent dwc3_request from being queued twice

Queueing the same request twice can introduce hard-to-debug
problems. At least one function driver - Android's f_mtp.c - is known
to cause this problem.

While that function is out-of-tree, this is a problem that's easy
enough to avoid.
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/gadget.c

@@ -1437,6 +1437,11 @@ static int __dwc3_gadget_ep_queue(struct dwc3_ep *dep, struct dwc3_request *req)
 				&req->request, req->dep->name))
 		return -EINVAL;
 
+	if (WARN(req->status < DWC3_REQUEST_STATUS_COMPLETED,
+				"%s: request %pK already in flight\n",
+				dep->name, &req->request))
+		return -EINVAL;
+
 	pm_runtime_get(dwc->dev);
 
 	req->request.actual	= 0;