xen: flag pvcalls-front to be not essential for system boot

The Xen pvcalls device is not essential for booting. Set the respective
flag.
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20211022064800.14978-5-jgross@suse.comSigned-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/pvcalls-front.c

@@ -1275,6 +1275,7 @@ static struct xenbus_driver pvcalls_front_driver = {
 	.probe = pvcalls_front_probe,
 	.remove = pvcalls_front_remove,
 	.otherend_changed = pvcalls_front_changed,
+	.not_essential = true,
 };
 
 static int __init pvcalls_frontend_init(void)