wifi: rtw89: rename sub_entity to chanctx

Originally, we planed to fill MAC_0/1 indicators with chanctx and
use sub_entity_xxx for these things. However, there are some reasons
listed below which make us give up this plan after we know our Wi-Fi 7
HW design.
	1. one link is bound to one HW band during its life time
	   but, one link might change chanctx dynamically
	2. in concurrent mode, assume 1st vif is MLD
	   1st vif's 2nd link might use the same chanctx as 2nd vif
	   but, they are not on the same HW band
So, we let sub_entity_xxx stuffs deal with only chanctx now. And, to be
more readable, we rename sub_entity related words to chanctx.

No logic is changed.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240727080650.12195-4-pkshih@realtek.com