AP_InertialSensor: MPU9250: publish gyro raw sample rate

So that delta angle calculation is enabled.

libraries/AP_InertialSensor/AP_InertialSensor_MPU9250.cpp

@@ -316,6 +316,7 @@ bool AP_InertialSensor_MPU9250::_init_sensor(AP_HAL::SPIDeviceDriver *spi)
     hal.scheduler->register_timer_process(FUNCTOR_BIND_MEMBER(&AP_InertialSensor_MPU9250::_poll_data, void));
 
     _set_accel_raw_sample_rate(_accel_instance, DEFAULT_SAMPLE_RATE);
+    _set_gyro_raw_sample_rate(_gyro_instance, DEFAULT_SAMPLE_RATE);
 
 #if MPU9250_DEBUG
     _dump_registers(_spi);