Make digitalPotWrite depend on HAS_DIGIPOTSS

Marlin/Marlin_main.cpp

@@ -5660,10 +5660,12 @@ inline void gcode_M907() {
    * M908: Control digital trimpot directly (M908 P<pin> S<current>)
    */
   inline void gcode_M908() {
+    #if HAS_DIGIPOTSS
       digitalPotWrite(
         code_seen('P') ? code_value() : 0,
         code_seen('S') ? code_value() : 0
       );
+    #endif
     #ifdef DAC_STEPPER_CURRENT
       dac_current_raw(
         code_seen('P') ? code_value_long() : -1,

Marlin/stepper.cpp

@@ -1174,19 +1174,18 @@ void quickStop() {
 
 #endif //BABYSTEPPING
 
-// From Arduino DigitalPotControl example
-void digitalPotWrite(int address, int value) {
-  #if HAS_DIGIPOTSS
+#if HAS_DIGIPOTSS
+
+  // From Arduino DigitalPotControl example
+  void digitalPotWrite(int address, int value) {
     digitalWrite(DIGIPOTSS_PIN, LOW); // take the SS pin low to select the chip
     SPI.transfer(address); //  send in the address and value via SPI:
     SPI.transfer(value);
     digitalWrite(DIGIPOTSS_PIN, HIGH); // take the SS pin high to de-select the chip:
     //delay(10);
-  #else
-    UNUSED(address);
-    UNUSED(value);
-  #endif
-}
+  }
+
+#endif //HAS_DIGIPOTSS
 
 // Initialize Digipot Motor Current
 void digipot_init() {

Marlin/stepper.h

@@ -62,7 +62,9 @@ extern block_t* current_block;  // A pointer to the block currently being traced
 
 void quickStop();
 
-void digitalPotWrite(int address, int value);
+#if HAS_DIGIPOTSS
+  void digitalPotWrite(int address, int value);
+#endif
 void microstep_ms(uint8_t driver, int8_t ms1, int8_t ms2);
 void microstep_mode(uint8_t driver, uint8_t stepping);
 void digipot_init();