Make digitalPotWrite depend on HAS_DIGIPOTSS

Marlin/Marlin_main.cpp

    * M908: Control digital trimpot directly (M908 P<pin> S<current>)
    */
   inline void gcode_M908() {
-    digitalPotWrite(
-      code_seen('P') ? code_value() : 0,
-      code_seen('S') ? code_value() : 0
-    );
+    #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

 
 #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

 
 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();