usb: musb: avoid null pointer dereference in debug logging

Since commit 511f3c53 (usb: gadget: udc-core: fix a regression during
gadget driver unbinding) usb_gadget_remove_driver will pass NULL for
the driver argument.
Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/musb/musb_gadget.c

@@ -1937,7 +1937,8 @@ static int musb_gadget_stop(struct usb_gadget *g,
 	stop_activity(musb, driver);
 	otg_set_peripheral(musb->xceiv->otg, NULL);
 
-	dev_dbg(musb->controller, "unregistering driver %s\n", driver->function);
+	dev_dbg(musb->controller, "unregistering driver %s\n",
+				  driver ? driver->function : "(removed)");
 
 	musb->is_active = 0;
 	musb->gadget_driver = NULL;