wil6210: Fix: Tx stall

Due to multi-tx-queue design, wil_start_xmit() used to be executed
concurrently for different queues. Then, these transmits delivered
to the same queue, creating race.

As result, Tx descriptor may be skipped, causing stall in hardware.

Convert to single Tx queue fixes it.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>