xen/manage: correct return value check on xenbus_scanf()

A negative return value indicates an error; in fact the function at
present won't ever return zero.
Signed-off-by: NJan Beulich <jbeulich@suse.com>
Reviewed-by: NJuergen Gross <jgross@suse.com>
Signed-off-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/manage.c

@@ -277,7 +277,7 @@ static void sysrq_handler(struct xenbus_watch *watch, const char **vec,
 	err = xenbus_transaction_start(&xbt);
 	if (err)
 		return;
-	if (!xenbus_scanf(xbt, "control", "sysrq", "%c", &sysrq_key)) {
+	if (xenbus_scanf(xbt, "control", "sysrq", "%c", &sysrq_key) < 0) {
 		pr_err("Unable to read sysrq code in control/sysrq\n");
 		xenbus_transaction_end(xbt, 1);
 		return;