CLEANUP: config: Return ERR_NONE from config callbacks instead of 0

Return ERR_NONE instead of 0 on success for all config callbacks that should
return ERR_* codes. There is no change because ERR_NONE is a macro equals to
0. But this makes the return value more explicit.
diff --git a/src/wurfl.c b/src/wurfl.c
index 06029aa..34075d6 100644
--- a/src/wurfl.c
+++ b/src/wurfl.c
@@ -263,7 +263,7 @@
 	// wurfl-data-file not configured, WURFL is not used so don't try to
 	// configure it.
 	if (global_wurfl.data_file == NULL)
-		return 0;
+		return ERR_NONE;
 
 	ha_notice("WURFL: Loading module v.%s\n", HA_WURFL_MODULE_VERSION);
 	// creating WURFL handler
@@ -395,7 +395,7 @@
 
 	ha_notice("WURFL: Engine loaded\n");
 	ha_notice("WURFL: Module load completed\n");
-	return 0;
+	return ERR_NONE;
 }
 
 static void ha_wurfl_deinit(void)