mt76: fix race condition in station removal

If there are still pending packets in the tx queue when removing a station,
it could possibly lead to a call to further attempts to pull packets from
the mac80211 tx queue after it has already been removed from the scheduling
list.
Prevent this from happening by calling synchronize_rcu after deleting the
wcid pointer before further cleaning up the tx queues.
To be extra careful, ensure that mtxq->list is always initialized properly.

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