ath10k: fix throughput regression in multi client mode

commit 7a0adc83 ("ath10k: improve tx scheduling") is causing
severe throughput drop in multi client mode. This issue is originally
reported in veriwave setup with 50 clients with TCP downlink traffic.
While increasing number of clients, the average throughput drops
gradually. With 50 clients, the combined peak throughput is decreased
to 98 Mbps whereas reverting given commit restored it to 550 Mbps.

Processing txqs for every tx completion is causing overhead. Ideally for
management frame tx completion, pending txqs processing can be avoided.
The change partly reverts the commit "ath10k: improve tx scheduling".
Processing pending txqs after all skbs tx completion will yeild enough
room to burst tx frames.

Fixes: 7a0adc83 ("ath10k: improve tx scheduling")
Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>