Fix Javadoc warnings

Change-Id: I857d8bd7d9fc0862f3c0097d3e65cbedfb997b0f
diff --git a/openflow/api/src/main/java/org/onosproject/openflow/controller/OpenFlowSwitch.java b/openflow/api/src/main/java/org/onosproject/openflow/controller/OpenFlowSwitch.java
index 3b8358f..11cb705 100644
--- a/openflow/api/src/main/java/org/onosproject/openflow/controller/OpenFlowSwitch.java
+++ b/openflow/api/src/main/java/org/onosproject/openflow/controller/OpenFlowSwitch.java
@@ -144,6 +144,8 @@
 
     /**
      * Identifies the channel used to communicate with the switch.
+     *
+     * @return string representation of the connection to the device
      */
     public String channelId();