wifi: mt76: mt7996: Rely on mt7996_sta_link in sta_add/sta_remove callbacks

Generalize mt7996_mac_sta_add() and mt7996_mac_sta_remove() routines to
deal with mt7996_sta_link structure.
This is a preliminary patch to introduce MLO support for MT7996 driver.

Co-developed-by: Bo Jiao <Bo.Jiao@mediatek.com>
Signed-off-by: Bo Jiao <Bo.Jiao@mediatek.com>
Co-developed-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Co-developed-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20250311-mt7996-mlo-v2-7-31df6972519b@kernel.org


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