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>