wifi: mt76: mt7996: Set def_wcid pointer in mt7996_mac_sta_init_link()

In order to get the ieee80211_sta pointer from wcid struct for a MLO
client, set def_wcid pointer in mt7996_mac_sta_init_link routine.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Tested-by: Jose Ignacio Tornos Martinez <jtornosm@redhat.com>
Link: https://patch.msgid.link/20250731-mt7996-mlo-devel-v1-1-7ff4094285d0@kernel.org


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