Copter: unify singleton naming to _singleton and get_singleton()

ArduCopter/toy_mode.cpp

@@ -957,7 +957,7 @@ void ToyMode::handle_message(mavlink_message_t *msg)
         AP_Notify::flags.video_recording = 1;
     } else if (strncmp(m.name, "WIFICHAN", 10) == 0) {
 #if HAL_RCINPUT_WITH_AP_RADIO
-        AP_Radio *radio = AP_Radio::instance();
+        AP_Radio *radio = AP_Radio::get_singleton();
         if (radio) {
             radio->set_wifi_channel(m.value);
         }