Tweak HAL_adc_start_conversion

Marlin/src/HAL/HAL_DUE/HAL_Due.cpp

 // ADC
 // --------------------------------------------------------------------------
 
-void HAL_adc_start_conversion(uint8_t adc_pin) {
+void HAL_adc_start_conversion(const uint8_t adc_pin) {
   HAL_adc_result = analogRead(adc_pin);
 }
 

Marlin/src/HAL/HAL_DUE/HAL_Due.h

 #define HAL_READ_ADC        HAL_adc_result
 
 
-void HAL_adc_start_conversion (uint8_t adc_pin);
+void HAL_adc_start_conversion(const uint8_t adc_pin);
 
 uint16_t HAL_adc_get_result(void);
 

Marlin/src/HAL/HAL_LPC1768/HAL.cpp

   };
 }
 
-void HAL_adc_start_conversion(uint8_t adc_pin) {
+void HAL_adc_start_conversion(const uint8_t adc_pin) {
   if (adc_pin >= (NUM_ANALOG_INPUTS) || adc_pin_map[adc_pin].port == 0xFF) {
     usb_serial.printf("HAL: HAL_adc_start_conversion: no pinmap for %d\n", adc_pin);
     return;

Marlin/src/HAL/HAL_LPC1768/HAL.h

 
 void HAL_adc_init(void);
 void HAL_adc_enable_channel(int pin);
-void HAL_adc_start_conversion (uint8_t adc_pin);
+void HAL_adc_start_conversion(const uint8_t adc_pin);
 uint16_t HAL_adc_get_result(void);
 
 #endif // _HAL_LPC1768_H

Marlin/src/HAL/HAL_TEENSY35_36/HAL_Teensy.h

 
 #define HAL_ANALOG_SELECT(pin) NOOP;
 
-void HAL_adc_start_conversion(uint8_t adc_pin);
+void HAL_adc_start_conversion(const uint8_t adc_pin);
 
 uint16_t HAL_adc_get_result(void);