usb: musb: gadget: do *unmap_dma_buffer* only for valid DMA addr

musb does not use DMA buffer for ep0 but it uses the same giveback
function *musb_g_giveback* for all endpoints (*musb_g_ep0_giveback* calls
*musb_g_giveback*). So for ep0 case request.dma will be '0'
and will result in kernel OOPS if tried to *unmap_dma_buffer* for requests in
ep0. Fixed it by doing *unmap_dma_buffer* only for valid DMA addr and
checking that musb_ep->dma is valid when unmapping.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/musb/musb_gadget.c

@@ -141,7 +141,9 @@ static inline void map_dma_buffer(struct musb_request *request,
 static inline void unmap_dma_buffer(struct musb_request *request,
 				struct musb *musb)
 {
-	if (!is_buffer_mapped(request))
+	struct musb_ep *musb_ep = request->ep;
+
+	if (!is_buffer_mapped(request) || !musb_ep->dma)
 		return;
 
 	if (request->request.dma == DMA_ADDR_INVALID) {
@@ -195,7 +197,10 @@ __acquires(ep->musb->lock)
 
 	ep->busy = 1;
 	spin_unlock(&musb->lock);
-	unmap_dma_buffer(req, musb);
+
+	if (!dma_mapping_error(&musb->g.dev, request->dma))
+		unmap_dma_buffer(req, musb);
+
 	if (request->status == 0)
 		dev_dbg(musb->controller, "%s done request %p,  %d/%d\n",
 				ep->end_point.name, request,