usb: dwc2: host: fix Wmaybe-uninitialized warning

Uninitialized char* causes a sparse build-warning, fix it up by
initializing it to NULL.
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc2/hcd.c

@@ -4492,8 +4492,8 @@ static void dwc2_dump_urb_info(struct usb_hcd *hcd, struct urb *urb,
 {
 #ifdef VERBOSE_DEBUG
 	struct dwc2_hsotg *hsotg = dwc2_hcd_to_hsotg(hcd);
-	char *pipetype;
-	char *speed;
+	char *pipetype = NULL;
+	char *speed = NULL;
 
 	dev_vdbg(hsotg->dev, "%s, urb %p\n", fn_name, urb);
 	dev_vdbg(hsotg->dev, "  Device address: %d\n",