wifi: rtw89: link rtw89_vif and chanctx stuffs

First, introduce struct rtw89_sub_entity for chanctx related stuffs.
Second, add enum rtw89_sub_entity_idx to rtw89_vif for vif operation
to access its/right chanctx stuffs after future multi-channel support.

Besides, RTW89_SUB_ENTITY_0 is the default chanctx entry throughout
driver, i.e. it's used for things which may not have a target chanctx
yet. So, we need to ensure that RTW89_SUB_ENTITY_0 is always working.
If there is at least one alive chanctx, then one of them must take
RTW89_SUB_ENTITY_0. If no alive chanctx, RTW89_SUB_ENTITY_0 will be
filled by rtw89_config_default_chandef().

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/20221129083130.45708-7-pkshih@realtek.com