adding the listener to the device service

Change-Id: I2ef4e222e8ac393e7880133c875097a507f47c7b
diff --git a/app/src/main/java/org/onosproject/olt/impl/Olt.java b/app/src/main/java/org/onosproject/olt/impl/Olt.java
index 11c1e5b..ce94c2b 100644
--- a/app/src/main/java/org/onosproject/olt/impl/Olt.java
+++ b/app/src/main/java/org/onosproject/olt/impl/Olt.java
@@ -150,6 +150,8 @@
                 .filter(p -> p.isEnabled())
                 .forEach(p -> installFilteringObjectives((DeviceId) p.element().id(), p));
 
+        deviceService.addListener(deviceListener);
+
         log.info("Started with Application ID {}", appId.id());
     }