wifi: rtw89: introduce helper to get designated link for MLO

wifi: rtw89: introduce helper to get designated link for MLO

A link bound to HW band 0 was previously always assumed to exist, because
it's true on non-MLD connection, and MLO connection is not supported yet.
Now, start to consider MLO cases and prepare to enable MLO support in the
following. Add skeleton of designated link. For single-link cases, helper
returns the one. For multi-link cases, priorities can be scheduled. Then,
drop assumption of link bound to HW band 0.

One exception is that MCC doesn't work with MLD yet, so it still expects
link on HW band 0 somewhere.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250428112456.13165-11-pkshih@realtek.com