Fix issues found by FindBugs: NP_TOSTRING_COULD_RETURN_NULL

http://findbugs.sourceforge.net/bugDescriptions.html#NP_TOSTRING_COULD_RETURN_NULL

If the DHCP Packet Type is not known / unrecognized, then return
a string like "DHCP_UnknownType(56)"

Change-Id: I8b97f890a882182cc6e2ab20d24a231b8558e509
diff --git a/src/main/java/net/onrc/onos/core/packet/DHCPPacketType.java b/src/main/java/net/onrc/onos/core/packet/DHCPPacketType.java
index 9d83c15..7c4cdb8 100644
--- a/src/main/java/net/onrc/onos/core/packet/DHCPPacketType.java
+++ b/src/main/java/net/onrc/onos/core/packet/DHCPPacketType.java
@@ -79,7 +79,7 @@
                 return "DHCPLEASEACTIVE";
         }
 
-        return null;
+        return "DHCP_UnknownType(" + value + ")";
     }
 
     public static DHCPPacketType getType(int value) {