Encapsulate MMU internal state.

Firmware/mmu.cpp

@@ -28,7 +28,7 @@ bool mmu_enabled = false;
 
 bool mmu_ready = false;
 
-int8_t mmu_state = 0;
+static int8_t mmu_state = 0;
 
 uint8_t mmu_cmd = 0;
 

Firmware/mmu.h

@@ -5,8 +5,6 @@
 
 extern bool mmu_enabled;
 
-extern int8_t mmu_state;
-
 extern uint8_t mmu_extruder;
 
 extern uint8_t tmp_extruder;