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/U32.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/U32.java
index 7f53374..c69786c 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/U32.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/U32.java
@@ -61,7 +61,7 @@
 
     @Override
     public String toString() {
-        return "" + f(raw);
+        return String.format("0x%08x", raw);
     }
 
     @Override