wifi: mt76: mt7996: fix crash on some tx status reports

When a wcid can't be found, link_sta can be stale from a previous batch.
The code currently assumes that if link_sta is set, wcid is also non-zero.
Fix wcid NULL pointer dereference by resetting link_sta when a wcid entry
can't be found.

Fixes: 62da647a ("wifi: mt76: mt7996: Add MLO support to mt7996_tx_check_aggr()")
Link: https://patch.msgid.link/20250827085352.51636-1-nbd@nbd.name


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