wl12xx: use ieee80211_free_txskb()

Use the newly introduced ieee80211_free_txskb() instead
of dev_kfree_skb() for failed tx packets.

Additionally, if the skb is a dummy packet, re-enqueue
it (as the fw expects it) instead of freeing it.

Reported-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>