wifi: rtw89: refactor VIF related func ahead for MLO

Refactor VIF related functions, e.g. add/remove/assoc/mapping
to separate most link stuffs into sub-functions for MLO reuse.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240916053158.47350-6-pkshih@realtek.com