rtw88: Use secondary channel offset enumeration

The hardware value of secondary channel offset isn't very intuitive. This
commit adds enumeration, so we can easier to check the logic with the
suffix of enumeration name, likes _UPPER or _LOWER.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Reviewed-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>