ath9k: Remove duplicate variables

A few variables (bssid, bssidmask, curaid) were duplicated in
struct ath_softc and in ath_hal, remove them.

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