Expose motor current in more configs, disable by default

Marlin/stepper.cpp

@@ -91,7 +91,7 @@ static volatile char endstop_hit_bits = 0; // use X_MIN, Y_MIN, Z_MIN and Z_MIN_
   #ifndef PWM_MOTOR_CURRENT
     #define PWM_MOTOR_CURRENT DEFAULT_PWM_MOTOR_CURRENT
   #endif
-  int motor_current_setting[3] = PWM_MOTOR_CURRENT;
+  const int motor_current_setting[3] = PWM_MOTOR_CURRENT;
 #endif
 
 static bool check_endstops = true;