ath9k: Fix function argument type

ath9k_vif_iter() was earlier used as an iterator
routine when calling a mac80211 utility. This is no
longer the case and hence we can mention the argument
type explicitly.

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