java_gen/U* types: change toString to hex

We mostly use the U* classes to represent non-numeric data.
Hex strings seem like a more useful representation.
diff --git a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/U16.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/U16.java
index 9de7e14..6c4ac11 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/U16.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/U16.java
@@ -63,7 +63,7 @@
 
     @Override
     public String toString() {
-        return Integer.toString(f(raw));
+        return String.format("0x%04x", raw);
     }
 
     @Override