wlcore: consider dummy packets when tx queues are empty

Don't ignore dummy packets when our queues are empty. This causes dummy
packets never to be sent when traffic is not suspended by FW thresholds,
which happens only in high Tx throughput situations. This may hurt Rx
performance.

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