ath10k: deduplicate bitrate to rate idx conversion

It's possible to derive rate index from bitrate
without any additional mapping structures/logic.

This should have little to none impact on
performance since this is only done for management
frames and the previous approach wasn't
particularly optimized.

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