MPLS-related bug fixes:
 - Use subtype() instead of type() inside method
   ModMplsLabelInstruction.toString()
 - Check whether the MPLS value is negative inside the MplsLabel()
   constructor.

Change-Id: I770194774219f0c919474928803e078226f2005d
diff --git a/core/api/src/main/java/org/onosproject/net/flow/instructions/L2ModificationInstruction.java b/core/api/src/main/java/org/onosproject/net/flow/instructions/L2ModificationInstruction.java
index 6147551..5135557 100644
--- a/core/api/src/main/java/org/onosproject/net/flow/instructions/L2ModificationInstruction.java
+++ b/core/api/src/main/java/org/onosproject/net/flow/instructions/L2ModificationInstruction.java
@@ -305,7 +305,7 @@
 
         @Override
         public String toString() {
-            return toStringHelper(type().toString())
+            return toStringHelper(subtype().toString())
                     .add("mpls", mplsLabel).toString();
         }