wl12xx: set the actual tid instead of the ac

When passing a tx frame, the driver incorrectly set desc->tid
with the ac instead of the actual tid.

It has some serious implications when using 802.11n + QoS,
as the fw starts a BlockAck with the wrong tid (which finally
cause beacon loss and disconnection / some fw crash)

Fix it by using the actual tid stored in skb->priority.

Reported-by: Shahar Levi <shahar_levi@ti.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>