ath9k: add AR9580 support

This has been tested in STA and AP mode by Florian.

Cc: David Quan <dquan@qca.qualcomm.com>
Cc: Kathy Giori <kgiori@qca.qualcomm.com>
Cc: Senthil Balasubramanian <senthilb@qca.qualcomm.com>
Tested-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>