ath9k: Free Beacon skbs in AP mode

Fix a memory leak where AP mode did not free transmitted Beacon frame
skbs.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Acked-by: Pat Erley <pat-lkml@erley.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>