mac80211: fix mgmt-tx abort cookie and leak

If a mgmt-tx operation is aborted before it runs, the wrong
cookie is reported back to userspace, and the ack_skb gets
leaked since the frame is freed directly instead of freeing
it using ieee80211_free_txskb(). Fix that.

Fixes: 3b79af97 ("mac80211: stop using pointers as userspace cookies")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>