Init all endstop bits to 0 (PR#2332)

Marlin/stepper.cpp

       #else
         byte
       #endif
-      current_endstop_bits;
+          current_endstop_bits = 0;
 
       #define _ENDSTOP_PIN(AXIS, MINMAX) AXIS ##_## MINMAX ##_PIN
       #define _ENDSTOP_INVERTING(AXIS, MINMAX) AXIS ##_## MINMAX ##_ENDSTOP_INVERTING