mc_att_control: keep integral enabled based on land detector

Previously the rate controller disabled updating the integral below 20%
throttle. This is not ideal for several reasons:
- some racers already hover with 20% throttle.
- for acro it is important to always keep the integral enabled, it has a
  noticeable effect on flight performance.

src/modules/mc_att_control/mc_att_control.hpp

@@ -55,6 +55,7 @@
 #include <uORB/topics/vehicle_control_mode.h>
 #include <uORB/topics/vehicle_rates_setpoint.h>
 #include <uORB/topics/vehicle_status.h>
+#include <uORB/topics/vehicle_land_detected.h>
 
 /**
  * Multicopter attitude control app start / stop handling function
@@ -99,6 +100,7 @@ private:
 	void		battery_status_poll();
 	void		parameter_update_poll();
 	void		sensor_bias_poll();
+	void		vehicle_land_detected_poll();
 	void		sensor_correction_poll();
 	void		vehicle_attitude_poll();
 	void		vehicle_attitude_setpoint_poll();
@@ -136,6 +138,7 @@ private:
 	int		_sensor_gyro_sub[MAX_GYRO_COUNT];	/**< gyro data subscription */
 	int		_sensor_correction_sub{-1};	/**< sensor thermal correction subscription */
 	int		_sensor_bias_sub{-1};		/**< sensor in-run bias correction subscription */
+	int		_vehicle_land_detected_sub{-1};	/**< vehicle land detected subscription */
 
 	unsigned _gyro_count{1};
 	int _selected_gyro{0};
@@ -160,6 +163,7 @@ private:
 	struct sensor_gyro_s			_sensor_gyro {};	/**< gyro data before thermal correctons and ekf bias estimates are applied */
 	struct sensor_correction_s		_sensor_correction {};	/**< sensor thermal corrections */
 	struct sensor_bias_s			_sensor_bias {};	/**< sensor in-run bias corrections */
+	struct vehicle_land_detected_s		_vehicle_land_detected {};
 
 	MultirotorMixer::saturation_status _saturation_status{};