mt76: fix possible NULL pointer dereference in mt76_tx

Even if this is not a real issue since mt76_tx is never run with wcid set
to NULL, fix a theoretical NULL pointer dereference in mt76_tx routine

Fixes: db9f11d3

 ("mt76: store wcid tx rate info in one u32 reduce locking")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>