xen: xen-pciback: fix warning Using plain integer as NULL pointer

Changes passing function argument 0 to NULL to avoid below sparse
warning

 CHECK   drivers/xen/xen-pciback/xenbus.c
drivers/xen/xen-pciback/xenbus.c:700:51: warning: Using plain integer as
NULL pointer
Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/xen-pciback/xenbus.c

@@ -697,7 +697,7 @@ static int xen_pcibk_xenbus_probe(struct xenbus_device *dev,
 	/* We need to force a call to our callback here in case
 	 * xend already configured us!
 	 */
-	xen_pcibk_be_watch(&pdev->be_watch, NULL, 0);
+	xen_pcibk_be_watch(&pdev->be_watch, NULL, NULL);
 
 out:
 	return err;