Fix for Kryo related issue crossing OSGi bundle boundaries.

Change-Id: I121dfe360de14a5b4760e5d2fd8cb2db93e0be63
diff --git a/core/store/src/main/java/org/onlab/onos/store/device/impl/PortNumberSerializer.java b/core/store/src/main/java/org/onlab/onos/store/device/impl/PortNumberSerializer.java
index 4483187..ae11867 100644
--- a/core/store/src/main/java/org/onlab/onos/store/device/impl/PortNumberSerializer.java
+++ b/core/store/src/main/java/org/onlab/onos/store/device/impl/PortNumberSerializer.java
@@ -8,9 +8,15 @@
 import com.esotericsoftware.kryo.io.Output;
 
 // TODO move to util, etc.
+/**
+ * Serializer for {@link PortNumber}.
+ */
 public final class PortNumberSerializer extends
         Serializer<PortNumber> {
 
+    /**
+     * Default constructor.
+     */
     public PortNumberSerializer() {
         // non-null, immutable
         super(false, true);