ath10k: fix tx hang

The wake_tx_queue/push_pending logic had a bug
which could stop queues indefinitely effectivelly
breaking traffic.

Fixes: 29946878 ("ath10k: implement wake_tx_queue")
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>