Fix L64xx init for Z4

Marlin/src/module/stepper/L64xx.cpp

   #if AXIS_IS_L64XX(Z3)
     L6470_INIT_CHIP(Z3);
   #endif
+  #if AXIS_IS_L64XX(Z4)
+    L6470_INIT_CHIP(Z4);
+  #endif
   #if AXIS_IS_L64XX(E0)
     L6470_INIT_CHIP(E0);
   #endif