wifi: mt76: fix vif link allocation

Reuse the vif deflink for link_id = 0 in order to avoid confusion with
vif->bss_conf, which also gets a link id of 0.

Link: https://patch.msgid.link/20250704-mt7996-mlo-fixes-v1-1-356456c73f43@kernel.org


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