AP_Camera: unify singleton naming to _singleton and get_singleton()

libraries/AP_Camera/AP_Camera.cpp

@@ -390,7 +390,7 @@ void AP_Camera::setup_feedback_callback(void)
 // log_picture - log picture taken and send feedback to GCS
 void AP_Camera::log_picture()
 {
-    AP_Logger *df = AP_Logger::instance();
+    AP_Logger *df = AP_Logger::get_singleton();
     if (df == nullptr) {
         return;
     }
@@ -437,7 +437,7 @@ void AP_Camera::update_trigger()
         _camera_trigger_logged = _camera_trigger_count;
 
         gcs().send_message(MSG_CAMERA_FEEDBACK);
-        AP_Logger *df = AP_Logger::instance();
+        AP_Logger *df = AP_Logger::get_singleton();
         if (df != nullptr) {
             if (df->should_log(log_camera_bit)) {
                 uint32_t tdiff = AP_HAL::micros() - timestamp32;