rt2x00: clear up IV's on key removal

After looking at code I realized that my previous fix
95844124 ("rt2x00: clear IV's on start to fix AP mode regression")
was incomplete. We can still have wrong IV's after re-keyring.
To fix that, clear up IV's also on key removal.

Fixes: 710e6cc1 ("rt2800: do not nullify initialization vector data")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
tested-by: Emil Karlson <jekarl@iki.fi>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>