wl12xx: AP-mode - prevent Tx to stale/invalid stations

Don't pollute the queues with Tx directed to invalid stations. This
can happen during recovery.

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