mt76: usb: avoid queue/status spinlocks while passing tx status to mac80211

As already done for pcie code in commit 79d1c94c ("mt76: avoid
queue/status spinlocks while passing tx status to mac80211") make sure
that no tx related spinlocks are taken during the ieee80211_tx_status call

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