menu: Don't highlight disabled entries

menu: Don't highlight disabled entries

There's an off-by-one bug in the menu input handling code which makes
it possible to highlight/select the last entry in a menu even if that
entry is disabled.

It should be noted that you can't actually *do* anything with the
disabled entry, i.e. edit or execute it. Still, allowing it to be
highlighted does look a little strange.

Reported-by: Matt Sephton <matt.sephton@gmail.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>