calibration_routines: fix 'Command denied during calibration' error message

The uorb subscribe logic got changed for queued topics with
https://github.com/PX4/Firmware/pull/9436, meaning an orb_subscribe will
return past messages as well now.

This lead to an error 'Command denied during calibration' for the previously
received calibration start command.

src/modules/commander/calibration_routines.cpp

@@ -799,7 +799,16 @@ calibrate_return calibrate_from_orientation(orb_advert_t *mavlink_log_pub,
 
 int calibrate_cancel_subscribe()
 {
-	return orb_subscribe(ORB_ID(vehicle_command));
+	int vehicle_command_sub = orb_subscribe(ORB_ID(vehicle_command));
+	if (vehicle_command_sub >= 0) {
+		// make sure we won't read any old messages
+		struct vehicle_command_s cmd;
+		bool update;
+		while (orb_check(vehicle_command_sub, &update) == 0 && update) {
+			orb_copy(ORB_ID(vehicle_command), vehicle_command_sub, &cmd);
+		}
+	}
+	return vehicle_command_sub;
 }
 
 void calibrate_cancel_unsubscribe(int cmd_sub)