wifi: rtw89: mcc: stop at a role holding chanctx

In general, MCC (multi-channel concurrency) stops when some chanctx is
unassigned. Originally, we let FW to stop at a fixed role. However, it
might be the one to be unassigned.

So, iterate MCC roles and select one which is still holding chanctx.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240727080650.12195-3-pkshih@realtek.com