Linear Advance: Only change E DIR when needed (#18744)

Marlin/src/module/stepper.cpp

     #if ENABLED(MIXING_EXTRUDER)
       // We don't know which steppers will be stepped because LA loop follows,
       // with potentially multiple steps. Set all.
-      if (LA_steps >= 0)
+      if (LA_steps > 0)
         MIXER_STEPPER_LOOP(j) NORM_E_DIR(j);
-      else
+      else if (LA_steps < 0)
         MIXER_STEPPER_LOOP(j) REV_E_DIR(j);
     #else
-      if (LA_steps >= 0)
+      if (LA_steps > 0)
         NORM_E_DIR(stepper_extruder);
-      else
+      else if (LA_steps < 0)
         REV_E_DIR(stepper_extruder);
     #endif