MC controller: Update use to new uORB API

src/modules/mc_pos_control/mc_pos_control_main.cpp

@@ -1447,7 +1447,7 @@ MulticopterPositionControl::task_main()
 						_control_mode.flag_control_velocity_enabled))) {
 			if (_att_sp_pub != nullptr && _vehicle_status.is_rotary_wing) {
 				orb_publish(ORB_ID(vehicle_attitude_setpoint), _att_sp_pub, &_att_sp);
-			} else if (_att_sp_pub <= 0 && _vehicle_status.is_rotary_wing){
+			} else if (_att_sp_pub == nullptr && _vehicle_status.is_rotary_wing){
 				_att_sp_pub = orb_advertise(ORB_ID(vehicle_attitude_setpoint), &_att_sp);
 			}
 		}