wifi: mt76: mt7996: Fix possible OOB access in mt7996_tx()

Fis possible Out-Of-Boundary access in mt7996_tx routine if link_id is
set to IEEE80211_LINK_UNSPECIFIED

Fixes: 3ce8acb8 ("wifi: mt76: mt7996: Update mt7996_tx to MLO support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20250704-mt7996-mlo-fixes-v1-6-356456c73f43@kernel.org


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