xen/pvcalls: Fix a check in pvcalls_front_remove()

bedata->ref can't be less than zero because it's unsigned.  This affects
certain error paths in probe.  We first set ->ref = -1 and then we set
it to a valid value later.

Fixes: 21968190 ("xen/pvcalls: connect to the backend")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/pvcalls-front.c

@@ -1103,7 +1103,7 @@ static int pvcalls_front_remove(struct xenbus_device *dev)
 			kfree(map);
 		}
 	}
-	if (bedata->ref >= 0)
+	if (bedata->ref != -1)
 		gnttab_end_foreign_access(bedata->ref, 0, 0);
 	kfree(bedata->ring.sring);
 	kfree(bedata);