Fix BTN_BACK

Followup to #9835

Marlin/src/lcd/ultralcd.h

@@ -121,7 +121,7 @@
 
     #if BUTTON_EXISTS(BACK)
       #define BLEN_D 3
-      #define EN_D BIT(BLEN_D)
+      #define EN_D _BV(BLEN_D)
       #define LCD_BACK_CLICKED (buttons & EN_D)
     #endif