ath10k: rework legacy rx rate decoding

Instead of using a hacky table and magic values
use supported band information advertised to
mac80211.

This may impact performance a little when dealing
with legacy rx rates depending on system
architecture. It's probably negligible.

This also fixes a highly theoretical corner case
when HT/VHT rates weren't reported correctly if
channel frequency wasn't known.

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