ath10k: unify tx mode and dispatch

There are a few different tx paths depending on
firmware and frame itself.

Creating a uniform decision will make it possible
to switch between different txmode easier, both
for testing and for future features as well.

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