dgram: make setBroadcast() conform to v0.4 API

- don't return a value

lib/dgram.js

@@ -223,11 +223,9 @@ Socket.prototype.address = function() {
 
 
 Socket.prototype.setBroadcast = function(arg) {
-  if (this._handle.setBroadcast((arg) ? 1 : 0) == -1) {
-      throw errnoException(errno, 'setBroadcast');
+  if (this._handle.setBroadcast((arg) ? 1 : 0)) {
+    throw errnoException(errno, 'setBroadcast');
   }
-
-  return true;
 };