Minor cleanup of set_bed_leveling_enabled

Marlin/Marlin_main.cpp

     #elif HAS_ABL
 
       if (enable != planner.abl_enabled) {
-        planner.abl_enabled = !planner.abl_enabled;
-        if (!planner.abl_enabled)
+        planner.abl_enabled = enable;
+        if (!enable)
           set_current_from_steppers_for_axis(
             #if ABL_PLANAR
               ALL_AXES