wifi: rtw89: read link_sta corresponding to the link

Tweak code to not always access sta->deflink directly. Instead,
according to link_id, read target link_sta from sta->link[].

For now, rtwsta_link->link_id keeps 0. When driver starts to
support MLO, the link_id will be assigned.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240916053158.47350-5-pkshih@realtek.com