AP_InertialSensor: disable fast sampling on 2nd IMU on Pixracer

it also hosts a mag, and gets lots of FIFO resets if we try to run it
at 8kHz

libraries/AP_InertialSensor/AP_InertialSensor.cpp

@@ -739,7 +739,8 @@ AP_InertialSensor::detect_backends(void)
         break;
 
     case AP_BoardConfig::PX4_BOARD_PIXRACER:
-        _fast_sampling_mask.set_default(3);
+        // only do fast samplng on ICM-20608. The MPU9250 doesn't handle high rate well when it has a mag enabled
+        _fast_sampling_mask.set_default(1);
         _add_backend(AP_InertialSensor_Invensense::probe(*this, hal.spi->get_device(HAL_INS_ICM20608_NAME), ROTATION_ROLL_180_YAW_90));
         _add_backend(AP_InertialSensor_Invensense::probe(*this, hal.spi->get_device(HAL_INS_MPU9250_NAME), ROTATION_ROLL_180_YAW_90));
         break;