wl18xx: count HW block spare based correctly on keys

We have no idea how many VIFs there are requiring a special spare, we
know just about the number of keys set. Rename the counter appropriately
and toggle it whenever a special key is added/removed.
Previously this was only changed once, since it was toggled whenever
the actual spare was changed.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>