Revert motor_current_setting to `constexpr`

We should use `constexpr` wherever it makes sense, for its added benefits. See http://stackoverflow.com/questions/13346879/const-vs-constexpr-on-variables

Marlin/stepper.h

       #ifndef PWM_MOTOR_CURRENT
         #define PWM_MOTOR_CURRENT DEFAULT_PWM_MOTOR_CURRENT
       #endif
-      static const int motor_current_setting[3] = PWM_MOTOR_CURRENT;
+      static constexpr int motor_current_setting[3] = PWM_MOTOR_CURRENT;
     #endif
 
     //