AP_InertialSensor: use fixed size arrays

apparently helps SITL build on MacOS

libraries/AP_InertialSensor/AP_InertialSensor.cpp

@@ -148,9 +148,9 @@ void
 AP_InertialSensor::_init_gyro()
 {
     uint8_t num_gyros = min(get_gyro_count(), INS_MAX_INSTANCES);
-    Vector3f last_average[num_gyros], best_avg[num_gyros];
-    float best_diff[num_gyros];
-    bool converged[num_gyros];
+    Vector3f last_average[INS_MAX_INSTANCES], best_avg[INS_MAX_INSTANCES];
+    float best_diff[INS_MAX_INSTANCES];
+    bool converged[INS_MAX_INSTANCES];
 
     // cold start
     hal.console->print_P(PSTR("Init Gyro"));
@@ -180,8 +180,8 @@ AP_InertialSensor::_init_gyro()
     // we try to get a good calibration estimate for up to 10 seconds
     // if the gyros are stable, we should get it in 1 second
     for (int16_t j = 0; j <= 20 && num_converged < num_gyros; j++) {
-        Vector3f gyro_sum[num_gyros], gyro_avg[num_gyros], gyro_diff[num_gyros];
-        float diff_norm[num_gyros];
+        Vector3f gyro_sum[INS_MAX_INSTANCES], gyro_avg[INS_MAX_INSTANCES], gyro_diff[INS_MAX_INSTANCES];
+        float diff_norm[INS_MAX_INSTANCES];
         uint8_t i;
 
         hal.console->print_P(PSTR("*"));
@@ -256,10 +256,10 @@ AP_InertialSensor::_init_accel()
 {
     uint8_t num_accels = min(get_accel_count(), INS_MAX_INSTANCES);
     uint8_t flashcount = 0;
-    Vector3f prev[num_accels];
-    Vector3f accel_offset[num_accels];
-    float total_change[num_accels];
-    float max_offset[num_accels];
+    Vector3f prev[INS_MAX_INSTANCES];
+    Vector3f accel_offset[INS_MAX_INSTANCES];
+    float total_change[INS_MAX_INSTANCES];
+    float max_offset[INS_MAX_INSTANCES];
 
     // cold start
     hal.scheduler->delay(100);
@@ -360,11 +360,11 @@ bool AP_InertialSensor::calibrate_accel(AP_InertialSensor_UserInteract* interact
                                         float &trim_pitch)
 {
     uint8_t num_accels = min(get_accel_count(), INS_MAX_INSTANCES);
-    Vector3f samples[num_accels][6];
-    Vector3f new_offsets[num_accels];
-    Vector3f new_scaling[num_accels];
-    Vector3f orig_offset[num_accels];
-    Vector3f orig_scale[num_accels];
+    Vector3f samples[INS_MAX_INSTANCES][6];
+    Vector3f new_offsets[INS_MAX_INSTANCES];
+    Vector3f new_scaling[INS_MAX_INSTANCES];
+    Vector3f orig_offset[INS_MAX_INSTANCES];
+    Vector3f orig_scale[INS_MAX_INSTANCES];
     uint8_t num_ok = 0;
 
     for (uint8_t k=0; k<num_accels; k++) {