wifi: rtw89: add subband index of primary channel to struct rtw89_chan

The subband index is a hardware value of relationship between primary
channel and bandwidth, and it is used by setting channel/bandwidth to
specify the primary channel.

Because this index is only needed when bandwidth >= 20 MHz, adjust
order of enumerator bandwidth to access offsets array easier. To prevent
misuse RTW89_CHANNEL_WIDTH_NUM as size, change it to
RTW89_CHANNEL_WIDTH_ORDINARY_NUM that will be the size of array. The
enumerator values of bandwidth (before ordinary number) will be also
used by upcoming TX power table built in firmware file, so add a comment
to remind keeping the order.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230920074322.42898-2-pkshih@realtek.com