wifi: mt76: mt7925: resolve primary mlink via def_wcid

wifi: mt76: mt7925: resolve primary mlink via def_wcid

Use mlink->wcid.def_wcid to obtain the primary mlink in
mt7925_mac_link_sta_add() instead of calling mt792x_sta_to_link().

The primary link context is already carried by the WCID, so the extra
lookup is redundant. This makes the add path follow the existing WCID
association directly.

No functional change intended.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Link: https://patch.msgid.link/20260306232238.2039675-6-sean.wang@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>