xen: flag xen_snd_front to be not essential for system boot

The Xen pv sound driver is not essential for booting. Set the respective
flag.

[boris: replace semicolon with comma]
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20211022064800.14978-6-jgross@suse.comSigned-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

sound/xen/xen_snd_front.c

@@ -358,6 +358,7 @@ static struct xenbus_driver xen_driver = {
 	.probe = xen_drv_probe,
 	.remove = xen_drv_remove,
 	.otherend_changed = sndback_changed,
+	.not_essential = true,
 };
 
 static int __init xen_drv_init(void)