PortNumber fixes

- hide new PortNumber(short) and use PortNumber.uint16 instead
- added utility to get PortNumber

Change-Id: Ia39fbe6e7126a3d9465d3035a06850c1d54a7f25
diff --git a/src/main/java/net/onrc/onos/core/util/PortNumber.java b/src/main/java/net/onrc/onos/core/util/PortNumber.java
index 0320a42..a9b42b0 100644
--- a/src/main/java/net/onrc/onos/core/util/PortNumber.java
+++ b/src/main/java/net/onrc/onos/core/util/PortNumber.java
@@ -35,7 +35,7 @@
      *
      * @param value the value to use.
      */
-    public PortNumber(short value) {
+    protected PortNumber(short value) {
         this.value = (int) shortToUnsignedLong(value);
     }