USB: core: hub: fix sparse warning

Fix the following sparse warning:

drivers/usb/core/hub.c:1664:37: warning: Using plain integer as NULL pointer
Signed-off-by: NFelipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/core/hub.c

@@ -1676,7 +1676,7 @@ static int usb_configure_device_otg(struct usb_device *udev)
 	if (!udev->bus->is_b_host
 			&& udev->config
 			&& udev->parent == udev->bus->root_hub) {
-		struct usb_otg_descriptor	*desc = 0;
+		struct usb_otg_descriptor	*desc = NULL;
 		struct usb_bus			*bus = udev->bus;
 
 		/* descriptor may appear anywhere in config */