ath6kl: Add functionality for starting AP mode

Use cfg80211 add/del_beacon callbacks for starting/stopping AP mode and
set_beacon to update AP configuration (mainly, to update Beacon and
Probe Response IEs).

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>