wifi: rtw89: make generic functions to convert subband gain index

The gain tables use different domain index, so we need to convert the index
from subband of chandef. Since these conversion functions can share with
8852b, make generic functions for further use.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221005083212.45683-6-pkshih@realtek.com