Mask out unused ATmega32U4 efuse bits (avrdudes#909)

Closes avrdudes#446

Author: MCUdude

src/avrdude.conf.in

@@ -13084,7 +13084,7 @@ part
     memory "efuse"
         size            = 1;
         write           = "1 0 1 0  1 1 0 0  1 0 1 0  0 1 0 0",
-                          "x x x x  x x x x  x x x x  i i i i";
+                          "x x x x  x x x x  1 1 1 1  i i i i";
 
         read            = "0 1 0 1  0 0 0 0  0 0 0 0  1 0 0 0",
                           "x x x x  x x x x  o o o o  o o o o";