stm32/system_stm32: Provide default value for HSI calibration.

If HSI is used the calibration value must be valid.  Fixes #4596.

ports/stm32/system_stm32.c

@@ -416,6 +416,7 @@ void SystemClock_Config(void)
     RCC_OscInitStruct.OscillatorType = MICROPY_HW_RCC_OSCILLATOR_TYPE;
     RCC_OscInitStruct.HSEState = MICROPY_HW_RCC_HSE_STATE;
     RCC_OscInitStruct.HSIState = MICROPY_HW_RCC_HSI_STATE;
+    RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
     #if defined(STM32H7)
     RCC_OscInitStruct.CSIState = RCC_CSI_OFF;
     #endif