xen/xenbus: fix xenbus_setup_ring()

Commit 4573240f ("xen/xenbus: eliminate xenbus_grant_ring()")
introduced an error for initialization of multi-page rings.

Cc: stable@vger.kernel.org
Fixes: 4573240f ("xen/xenbus: eliminate xenbus_grant_ring()")
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Signed-off-by: Juergen Gross <jgross@suse.com>