ath10k: allow dynamic bandwidth in hw rate control

Frames were never retransmitted with
different/lower bandwidths, e.g. only 80MHz
bandwidth was used when dealing with a VHT80
peer.

Allow HW rate control to try out different
bandwidths when retransmitting. This increases
robustness.

Also, document the parameter properly.

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