ath9k_htc: Fix station flags

The FW does absolutely nothing with the station flags,
so remove them. But keep the field around since it might
come in handy in the future.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>