ath10k: workaround qos nullfunc bug

ath10k: workaround qos nullfunc bug

Apparently fw/hw generates a corrupted QoS Control
Field in Qos NullFunc frames. The only way to
workaround this is to downgrade frames to
NullFunc. This should be okay since powersave is
done by fw/hw and these frames are only used for
CQM purposes (e.g. from hostapd to check if
station is still connected).

This doesn't fix any user visible bug that I know
of. It just prevents from sending out funky frames
on the air.

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