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>
ath10k: detect the number of spatial streams supported by hw

Until now ath10k assumed 3 spatial streams.
However some devices support only 2 spatial
streams.

This patch improves performance on devices that
don't support 3 spatial streams.

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