ath10k: improve tx throughput on slow machines

It is more efficient to move just the 802.11
header instead of the whole payload in most cases.

This has no measurable effect on modern hardware.
It should improve performance by a few percent on
hardware such as an Access Point that have a slow
CPU compared to a typical desktop CPU.

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