ekf2: fix IMU missed perf count when not using multi-EKF

src/modules/ekf2/EKF2.cpp

@@ -424,7 +424,7 @@ void EKF2::Run()
 		}
 
 	} else {
-		const unsigned last_generation = _vehicle_imu_sub.get_last_generation();
+		const unsigned last_generation = _sensor_combined_sub.get_last_generation();
 		sensor_combined_s sensor_combined;
 		imu_updated = _sensor_combined_sub.update(&sensor_combined);