usb: Don't init pointer to zero, but NULL

The pointer should always be inited to NULL, not zero (0). These are
two different things and not necessarily equal.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Stephen Warren <swarren@nvidia.com>

common/usb.c

@@ -1064,7 +1064,7 @@ static int usb_prepare_device(struct usb_device *dev, int addr, bool do_read,
 
 int usb_select_config(struct usb_device *dev)
 {
-	unsigned char *tmpbuf = 0;
+	unsigned char *tmpbuf = NULL;
 	int err;
 
 	err = get_descriptor_len(dev, USB_DT_DEVICE_SIZE, USB_DT_DEVICE_SIZE);