settings: use PDI constants

Use PDI constants where applicable.

Tested: Inspection only.

Change-Id: I2004b17c40dc94c8dab6dd43a1add46f873dad8d
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/settings.cpp b/settings.cpp
index 0445092..733ce9d 100644
--- a/settings.cpp
+++ b/settings.cpp
@@ -24,7 +24,7 @@
 
     auto mapperCall = bus.new_method_call(
         ObjectMapper::default_service, ObjectMapper::instance_path,
-        ObjectMapper::interface, "GetSubTree");
+        ObjectMapper::interface, ObjectMapper::method_names::get_sub_tree);
     mapperCall.append(root);
     mapperCall.append(depth);
     mapperCall.append(settingsIntfs);
@@ -102,7 +102,7 @@
     using Interfaces = std::vector<Interface>;
     auto mapperCall = bus.new_method_call(
         ObjectMapper::default_service, ObjectMapper::instance_path,
-        ObjectMapper::interface, "GetObject");
+        ObjectMapper::interface, ObjectMapper::method_names::get_object);
     mapperCall.append(path);
     mapperCall.append(Interfaces({interface}));