rtw89: phy: handle txpwr lmt/lmt_ru of 6G band

Add declarations of 6G stuff and extend rtw89_channel_to_idx() to
map 6G's channels to 6G channel indexes. While 6G, correspondingly
read 6G's entry for tx power limit and limit_ru.

After this, we should pay attention to chip_info::support_bands.
If a chip declares 6G support, it must configure txpwr_lmt_6g and
txpwr_lmt_ru_6g in case accessing NULL pointer while setting tx power
limit/limit_ru on 6G band.

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/20220218034017.9160-2-pkshih@realtek.com