xen-netback: prefer xenbus_scanf() over xenbus_gather()

For single items being collected this should be preferred as being more
typesafe (as the compiler can check format string and to-be-written-to
variable match) and more efficient (requiring one less parameter to be
passed).
Signed-off-by: NJan Beulich <jbeulich@suse.com>
Reviewed-by: NPaul Durrant <paul.durrant@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/xenbus.c

@@ -889,16 +889,16 @@ static int connect_ctrl_ring(struct backend_info *be)
 	unsigned int evtchn;
 	int err;
 
-	err = xenbus_gather(XBT_NIL, dev->otherend,
-			    "ctrl-ring-ref", "%u", &val, NULL);
-	if (err)
+	err = xenbus_scanf(XBT_NIL, dev->otherend,
+			   "ctrl-ring-ref", "%u", &val);
+	if (err < 0)
 		goto done; /* The frontend does not have a control ring */
 
 	ring_ref = val;
 
-	err = xenbus_gather(XBT_NIL, dev->otherend,
-			    "event-channel-ctrl", "%u", &val, NULL);
-	if (err) {
+	err = xenbus_scanf(XBT_NIL, dev->otherend,
+			   "event-channel-ctrl", "%u", &val);
+	if (err < 0) {
 		xenbus_dev_fatal(dev, err,
 				 "reading %s/event-channel-ctrl",
 				 dev->otherend);