mac80211: fix purging multicast PS buffer queue

commit 6b07d9ca upstream.

The code currently assumes that buffered multicast PS frames don't have
a pending ACK frame for tx status reporting.
However, hostapd sends a broadcast deauth frame on teardown for which tx
status is requested. This can lead to the "Have pending ack frames"
warning on module reload.
Fix this by using ieee80211_free_txskb/ieee80211_purge_tx_queue.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>