rtw89: convert rtw89_band to nl80211_band precisely

Before 6 GHz band was supported, i.e. only 2 GHz and 5 GHz, they were the
same from the numerical point of view. However, after 6 GHz band support,
we need to do this conversion logically.

Signed-off-by: Zong-Zhe Yang <kevin_yang@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/20220516005215.5878-6-pkshih@realtek.com