AP_InertialSensor: fixed pixfalcon 9250 detection

libraries/AP_InertialSensor/AP_InertialSensor.cpp

@@ -689,7 +689,10 @@ AP_InertialSensor::detect_backends(void)
         _add_backend(AP_InertialSensor_MPU6000::probe(*this, hal.spi->get_device(HAL_INS_MPU60x0_NAME)));
 
     } else if (AP_BoardConfig::get_board_type() == AP_BoardConfig::PX4_BOARD_PIXHAWK) {
-        _add_backend(AP_InertialSensor_MPU6000::probe(*this, hal.spi->get_device(HAL_INS_MPU60x0_NAME), ROTATION_ROLL_180));
+        if (!_add_backend(AP_InertialSensor_MPU6000::probe(*this, hal.spi->get_device(HAL_INS_MPU60x0_NAME), ROTATION_ROLL_180))) {
+            // handle pixfalcon with mpu9250 instead of mpu6000
+            _add_backend(AP_InertialSensor_MPU9250::probe(*this, hal.spi->get_device(HAL_INS_MPU9250_NAME), ROTATION_ROLL_180));
+        }
         _add_backend(AP_InertialSensor_LSM9DS0::probe(*this,
                                                       hal.spi->get_device(HAL_INS_LSM9DS0_G_NAME),
                                                       hal.spi->get_device(HAL_INS_LSM9DS0_A_NAME),