wifi: mt76: add def_wcid to struct mt76_wcid

add def_wcid to struct mt76_wcid to allow per-link wcid to retrieve the
struct ieee80211_sta structure.

Co-developed-by: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
Signed-off-by: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
Co-developed-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Link: https://patch.msgid.link/d5188ca633666eaad7e90a8061d1a0585487e128.1720248331.git.sean.wang@kernel.org


Signed-off-by: Felix Fietkau <nbd@nbd.name>