drm/xen-front: Fix loop timeout

If the loop times out then we want to exit with "to" set to zero, but in
the current code it's set to -1.

Fixes: c575b7eeb8 ("drm/xen-front: Add support for Xen PV display frontend")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180508092829.GC661@mwanda

drivers/gpu/drm/xen/xen_drm_front.c

@@ -778,7 +778,7 @@ static int xen_drv_remove(struct xenbus_device *dev)
 	 */
 	while ((xenbus_read_unsigned(front_info->xb_dev->otherend, "state",
 				     XenbusStateUnknown) != XenbusStateInitWait) &&
-				     to--)
+				     --to)
 		msleep(10);
 
 	if (!to) {