wifi: mt76: mt7996: Move num_sta accounting in mt7996_mac_sta_{add,remove}_links

Move phy num_sta accounting in mt7996_mac_sta_add() and
mt7996_mac_sta_remove() routines in order to take into account all
possibles MLO links.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20250704-mt7996-mlo-fixes-v1-9-356456c73f43@kernel.org


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