ath6kl: avoid flush_scheduled_work() usage

As per commit c4f135d6 ("workqueue: Wrap flush_workqueue() using
a macro") says, use per "struct ath6kl_usb" workqueue.

This is a blind conversion, and is only compile tested.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/f78ddbdc-8989-a1a7-2234-ce9ec3894625@I-love.SAKURA.ne.jp