AP_InertialSensor: MPU6000: publish sample rate

So that vibration and clipping may be calculated.

libraries/AP_InertialSensor/AP_InertialSensor_MPU6000.cpp

@@ -625,6 +625,8 @@ void AP_InertialSensor_MPU6000::start()
     _gyro_instance = _imu.register_gyro();
     _accel_instance = _imu.register_accel();
 
+    _set_accel_sample_rate(_accel_instance, 1000);
+
     hal.scheduler->resume_timer_procs();
 
     // start the timer process to read samples