ath9k: deny new interface addtion on IBSS mode

The present check denies the IBSS interface addtion if we
already have any other vifs. But it fails to deny interface
addition if IBSS was already present.

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