Cosmetic fix for VLAN_POP instruction

Change-Id: I224916db270d080df5c86da095e7b85f2b2948be
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 bfcc588..31bd2c0 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
@@ -309,7 +309,10 @@
 
         @Override
         public String toString() {
-            return subtype().toString() + SEPARATOR + ethernetType;
+            // etherType is not specified in VLAN_POP case. Ignore it.
+            return subtype().equals(L2SubType.VLAN_POP) ?
+                    subtype().toString() :
+                    subtype().toString() + SEPARATOR + ethernetType;
         }
 
         @Override