mt76: fix memory leak if device probing fails

Run mt76_free_device instead of ieee80211_free_hw if device probing
fails in order to remove the already allocated mt76 workqueue

Fixes: a86f1d01 ("mt76: move mt76 workqueue in common code")
Fixes: f1d96236

 ("mt76: mt7915: implement HE per-rate tx power support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>