wifi: rtw89: get designated link to replace link instance 0

wifi: rtw89: get designated link to replace link instance 0

Clean up some places where still to get link instance 0 directly.
Since now MLSR switch is supported, it's not guaranteed to always
run on link instance 0. So, prefer to get designated link in most
cases.

For now, the only exception is MCC (multi-channel concurrency) case.
How to fill content of its H2C command depends on how to choose link
instance, so cannot simply change it as above. Will handle MCC case
separately afterwards.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20260123013957.16418-10-pkshih@realtek.com