Fix an unused var warning

Marlin/src/HAL/STM32/HAL.cpp

@@ -133,6 +133,6 @@ void HAL_adc_start_conversion(const uint8_t adc_pin) { HAL_adc_result = analogRe
 
 uint16_t HAL_adc_get_result() { return HAL_adc_result; }
 
-void flashFirmware(int16_t) { NVIC_SystemReset(); }
+void flashFirmware(const int16_t) { NVIC_SystemReset(); }
 
 #endif // ARDUINO_ARCH_STM32 && !STM32GENERIC

Marlin/src/HAL/STM32/HAL.h

@@ -223,4 +223,4 @@ uint16_t HAL_adc_get_result();
 #define PARSED_PIN_INDEX(code, dval) parser.intval(code, dval)
 
 #define PLATFORM_M997_SUPPORT
-void flashFirmware(int16_t value);
+void flashFirmware(const int16_t);