wifi: rtw89: adjust channel encoding to common function

Since the range of channel table is identical among ICs. Make channel
encode/decode function common and not IC dependent. So all ICs with
matching firmware that needs this kind of coding can use it directly.
This patch doesn't change logic at all.

Signed-off-by: Po-Hao Huang <phhuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230220070202.29868-4-pkshih@realtek.com