Made MPU-6000 gyro optional

apps/drivers/mpu6000/mpu6000.cpp

@@ -446,8 +446,13 @@ MPU6000::init()
 	// write_reg(MPUREG_PWR_MGMT_1,MPU_CLK_SEL_PLLGYROZ);
 	usleep(1000);
 
-	/* do CDev init for the gyro device node */
-	ret = _gyro->init();
+	/* do CDev init for the gyro device node, keep it optional */
+	int gyro_ret = _gyro->init();
+
+	if (gyro_ret != OK) {
+		::close(_gyro_topic);
+		_gyro_topic = -1;
+	}
 
 	return ret;
 }
@@ -938,7 +943,9 @@ MPU6000::measure()
 
 	/* and publish for subscribers */
 	orb_publish(ORB_ID(sensor_accel), _accel_topic, &_accel_report);
-	orb_publish(ORB_ID(sensor_gyro), _gyro_topic, &_gyro_report);
+	if (_gyro_topic != -1) {
+		orb_publish(ORB_ID(sensor_gyro), _gyro_topic, &_gyro_report);
+	}
 
 	/* stop measuring */
 	perf_end(_sample_perf);