ath9k: Enable U-APSD AP mode support

mac80211 handles the actual operations, so ath9k can just indicate
support for this. Based on initial tests, this combination seems to
work fine.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>