Fix MK2_MULTIPLEXER stepper indirection

Addressing #8544

Marlin/stepper_indirection.h

@@ -429,6 +429,10 @@
     #define NORM_E_DIR() { switch (current_block->active_extruder) { case 0: E0_DIR_WRITE(!INVERT_E0_DIR); break; case 1: E0_DIR_WRITE(INVERT_E0_DIR); break; case 2: E1_DIR_WRITE(!INVERT_E1_DIR); break; case 3: E1_DIR_WRITE(INVERT_E1_DIR); } }
     #define REV_E_DIR() { switch (current_block->active_extruder) { case 0: E0_DIR_WRITE(INVERT_E0_DIR); break; case 1: E0_DIR_WRITE(!INVERT_E0_DIR); break; case 2: E1_DIR_WRITE(INVERT_E1_DIR); break; case 3: E1_DIR_WRITE(!INVERT_E1_DIR); } }
   #endif
+#elif ENABLED(MK2_MULTIPLEXER) // Even-numbered steppers are reversed
+  #define E_STEP_WRITE(v) E0_STEP_WRITE(v)
+  #define NORM_E_DIR() E0_DIR_WRITE(TEST(current_block->active_extruder, 0) ? !INVERT_E0_DIR: INVERT_E0_DIR)
+  #define REV_E_DIR() E0_DIR_WRITE(TEST(current_block->active_extruder, 0) ? INVERT_E0_DIR: !INVERT_E0_DIR)
 #elif EXTRUDERS > 4
   #define E_STEP_WRITE(v) { switch (current_block->active_extruder) { case 0: E0_STEP_WRITE(v); break; case 1: E1_STEP_WRITE(v); break; case 2: E2_STEP_WRITE(v); break; case 3: E3_STEP_WRITE(v); break; case 4: E4_STEP_WRITE(v); } }
   #define NORM_E_DIR() { switch (current_block->active_extruder) { case 0: E0_DIR_WRITE(!INVERT_E0_DIR); break; case 1: E1_DIR_WRITE(!INVERT_E1_DIR); break; case 2: E2_DIR_WRITE(!INVERT_E2_DIR); break; case 3: E3_DIR_WRITE(!INVERT_E3_DIR); break; case 4: E4_DIR_WRITE(!INVERT_E4_DIR); } }
@@ -472,14 +476,8 @@
   #endif
 #else
   #define E_STEP_WRITE(v) E0_STEP_WRITE(v)
-  #if ENABLED(MK2_MULTIPLEXER)
-    // Even-numbered steppers are reversed
-    #define NORM_E_DIR() E0_DIR_WRITE(TEST(current_block->active_extruder, 0) ? !INVERT_E0_DIR: INVERT_E0_DIR)
-    #define REV_E_DIR() E0_DIR_WRITE(TEST(current_block->active_extruder, 0) ? INVERT_E0_DIR: !INVERT_E0_DIR)
-  #else
   #define NORM_E_DIR() E0_DIR_WRITE(!INVERT_E0_DIR)
   #define REV_E_DIR() E0_DIR_WRITE(INVERT_E0_DIR)
 #endif
-#endif
 
 #endif // STEPPER_INDIRECTION_H