mt76x02: fix hdr pointer in write txwi for USB

Since we add txwi at the begining of skb->data, it no longer point
to ieee80211_hdr. This breaks settings TS bit for probe response and
beacons.

Acked-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>