ath11k: fix transmit queue flushing through flush_sta implementation

ath11k: fix transmit queue flushing through flush_sta implementation

warning print "ath11k c000000.wifi: failed to flush transmit queue 0"
is observed during busy times.

The mac80211 fallback implementation of `flush_sta` does not handle the per STA queues well.
This is fixed by providing a ath11k specific implementation of flush_sta telling the firmware to flush a given station.
The draining of the transmit queues should therefore stop correctly, even if new packets arrive in the mean time.

An upstream ath11k RFC is available at:
https://patchwork.kernel.org/project/linux-wireless/patch/GV1P250MB14333A5BF24623C4753A10E1E8E0A@GV1P250MB1433.EURP250.PROD.OUTLOOK.COM/

The patch was tested on a Xiaomi AX3600.

Signed-off-by: Florian Maurer <[email protected]>
Tested-by: Florian Maurer  <[email protected]>
Co-authored-by: Benjamin Berg <[email protected]>
Tested-by: Flole <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20293
Signed-off-by: Christian Marangi <[email protected]>