ath10k: fix pull-push tx threshold handling

This prevents tx hangs or hiccups if pull-push
supporting firmware defines per-txq thresholds or
switches modes dynamically.

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>