ath10k: simplify tx helpers

It always bugged me how tid is computed and stored
in a temporary var before written to the control
buffer. It was confusing and it made it difficult
to work with tx helpers.

While at it rename the qos workaround function as
it was misleading - it's not a workaround but
preparation for nwifi tx mode.

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