rtw89: extend subband for 6G band

Split 6G band into 8 sub-bands where indexes are from 0 to 7,
i.e. RTW89_CH_6G_BAND_IDX[0-7]. Then, decide subband by both
band and channel instead of just channel because conflicts
between 5G channels and 6G channels.

Moreover, add default case to the existing use of switch (subband).

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/20220211075953.40421-4-pkshih@realtek.com