wifi: mt76: mt7996: Fix valid_links bitmask in mt7996_mac_sta_{add,remove}

sta->valid_links bitmask can be set even for non-MLO client.

Fixes: dd82a9e0 ("wifi: mt76: mt7996: Rely on mt7996_sta_link in sta_add/sta_remove callbacks")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20250704-mt7996-mlo-fixes-v1-7-356456c73f43@kernel.org


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