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/51d.c b/src/51d.c
index 483a17b..31d7c3f 100644
--- a/src/51d.c
+++ b/src/51d.c
@@ -625,7 +625,7 @@
 	fiftyoneDegreesDataSetInitStatus _51d_dataset_status = DATA_SET_INIT_STATUS_NOT_SET;
 
 	if (!global_51degrees.data_file_path)
-		return 0;
+		return ERR_NONE;
 
 	if (global.nbthread < 1) {
 		ha_alert("51Degrees: The thread count cannot be zero or negative.\n");
@@ -708,7 +708,7 @@
 	}
 #endif
 
-	return 0;
+	return ERR_NONE;
 }
 
 static void deinit_51degrees(void)