df_mpu9250_wrapper: use the elapsed counter

src/platforms/posix/drivers/df_mpu9250_wrapper/df_mpu9250_wrapper.cpp

@@ -505,6 +505,8 @@ int DfMpu9250Wrapper::_publish(struct imu_sensor_data &data)
 		return 0;
 	}
 
+	perf_begin(_publish_perf);
+
 	_publish_count = 0;
 
 	// TODO: get these right
@@ -552,6 +554,8 @@ int DfMpu9250Wrapper::_publish(struct imu_sensor_data &data)
 		DevMgr::updateNotify(*this);
 	}
 
+	perf_end(_publish_perf);
+
 	// TODO: check the return codes of this function
 	return 0;
 };