Move HAS_DIGIPOTSS to conditionals

Marlin/Conditionals.h

   #define HAS_E2_STEP (PIN_EXISTS(E2_STEP))
   #define HAS_E3_STEP (PIN_EXISTS(E3_STEP))
   #define HAS_E4_STEP (PIN_EXISTS(E4_STEP))
+  #define HAS_DIGIPOTSS (PIN_EXISTS(DIGIPOTSS))
 
   #define HAS_MOTOR_CURRENT_PWM (PIN_EXISTS(MOTOR_CURRENT_PWM_XY) || PIN_EXISTS(MOTOR_CURRENT_PWM_Z) || PIN_EXISTS(MOTOR_CURRENT_PWM_E))
 

Marlin/pins.h

     X_MS1_PIN, X_MS2_PIN, Y_MS1_PIN, Y_MS2_PIN, Z_MS1_PIN, Z_MS2_PIN \
   }
 
-#define HAS_DIGIPOTSS (PIN_EXISTS(DIGIPOTSS))
-
 #endif //__PINS_H