wifi: rtw89: use struct to access RA report

RA (rate adaptive), a mechanism to select proper rate, is implemented in
firmware, and this report is used to tell driver TX rate it is currently
using. Use struct to access this report, and doesn't change logic at all.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230728070252.66525-8-pkshih@realtek.com