ports/itsybitsy5v: Only program high fuse byte

Leave the other two alone as they control brown-out and clock
configuration bits.

Signed-off-by: Keith Packard <keithp@keithp.com>

ports/itsybitsy5v/snek-itsybitsy-install.in

@@ -54,7 +54,7 @@ for i in "$@"; do
     esac
 done
 
-FUSES="-U lfuse:w:0xff:m -U hfuse:w:0x91:m -U efuse:w:0xfd:m"
+FUSES="-U hfuse:w:0x99:m"
 
 case "$action" in
     fuse)