ath10k: fix FW crashes on heavy TX on 10.1.389 AP FW

10.1.389 firmware has some differences in
calculation of number of outstanding HTT TX
completions. This led to FW crashes of 10.1.389
while main firmware branch was unnaffected.

The patch makes sure ath10k doesn't queue up more
MSDUs than it should.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>