usb: dwc3: gadget: do not map/unmap ZLP transfers

If the gadget drivers sends a ZLP we are trying to map this this request
which does not work on all implementations. So we simply skip mapping
it.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/gadget.c

@@ -61,6 +61,11 @@ void dwc3_map_buffer_to_dma(struct dwc3_request *req)
 {
 	struct dwc3			*dwc = req->dep->dwc;
 
+	if (req->request.length == 0) {
+		/* req->request.dma = dwc->setup_buf_addr; */
+		return;
+	}
+
 	if (req->request.dma == DMA_ADDR_INVALID) {
 		req->request.dma = dma_map_single(dwc->dev, req->request.buf,
 				req->request.length, req->direction
@@ -78,6 +83,11 @@ void dwc3_unmap_buffer_from_dma(struct dwc3_request *req)
 {
 	struct dwc3			*dwc = req->dep->dwc;
 
+	if (req->request.length == 0) {
+		req->request.dma = DMA_ADDR_INVALID;
+		return;
+	}
+
 	if (req->mapped) {
 		dma_unmap_single(dwc->dev, req->request.dma,
 				req->request.length, req->direction