wifi: mt76: mt7996: Account active links in valid_links fields

wifi: mt76: mt7996: Account active links in valid_links fields

Track active vif links in mt7996_vif_link_add and mt7996_vif_link_remove
routines.
This is a preliminary patch in order to remove AP MLD links from MCU
configuration during AP tear-down process and to support MLO link
reconfiguration in MT7996 driver.

Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Co-developed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260315-mt7996-mlo-link-reconf-v1-2-a8a634fbc927@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>