mqttdpms: Retain online availability message

The *online* message sent to the availability topic needs to have the
retained flag set.  Without it, when Home Assistant restarts, it will
receive the retained *offline* message and mark the entity as
unavailable.

src/mqttdpms.rs

@@ -170,7 +170,7 @@ impl MqttDpms {
             .topics
             .availability
             .replace("@UNIQUEID@", &self.config.unique_id);
-        let message = mqtt::Message::new(topic, "online", 0);
+        let message = mqtt::Message::new_retained(topic, "online", 0);
         Ok(client.publish(message).await?)
     }