Fixing a defect in simulated topology to make sure to use unique ports.

Change-Id: I6ae266e9347470722e4df31aed18e3141e3e84a4
diff --git a/providers/null/src/main/java/org/onosproject/provider/nil/LinearTopologySimulator.java b/providers/null/src/main/java/org/onosproject/provider/nil/LinearTopologySimulator.java
index 8e8c759..beb06c3 100644
--- a/providers/null/src/main/java/org/onosproject/provider/nil/LinearTopologySimulator.java
+++ b/providers/null/src/main/java/org/onosproject/provider/nil/LinearTopologySimulator.java
@@ -42,8 +42,10 @@
 
     @Override
     protected void createLinks() {
+        int portOffset = 1;
         for (int i = 0, n = deviceCount - 1; i < n; i++) {
-            createLink(i, i + 1);
+            createLink(i, i + 1, portOffset, 1);
+            portOffset = 2;
         }
     }