Conditionals for SWITCHING_EXTRUDER

In reference to #5367. SWITCHING_EXTRUDER was not fully implemented for EXTRUDERS > 2.

Marlin/src/inc/Conditionals_LCD.h

@@ -421,12 +421,22 @@
   #define HOTEND_INDEX  e
 #endif
 
-#if ENABLED(SWITCHING_EXTRUDER) || ENABLED(MIXING_EXTRUDER)   // Unified E axis
-  #if ENABLED(MIXING_EXTRUDER)
-    #define E_STEPPERS  MIXING_STEPPERS
+#if ENABLED(SWITCHING_EXTRUDER)                               // One stepper for every two EXTRUDERS
+  #if EXTRUDERS > 4
+    #define E_STEPPERS    3
+    #define E_MANUAL      3
+    #define TOOL_E_INDEX  current_block->active_extruder
+  #elif EXTRUDERS > 2
+    #define E_STEPPERS    2
+    #define E_MANUAL      2
+    #define TOOL_E_INDEX  current_block->active_extruder
   #else
-    #define E_STEPPERS  1                                     // One E stepper
+    #define E_STEPPERS    1
+    #define TOOL_E_INDEX  0
   #endif
+  #define E_MANUAL        E_STEPPERS
+#elif ENABLED(MIXING_EXTRUDER)
+  #define E_STEPPERS      MIXING_STEPPERS
   #define E_MANUAL        1
   #define TOOL_E_INDEX    0
 #else