ath9k: update hw configuration for virtual wiphys

ath9k supports its own virtual wiphys. The hardware code
relies on the ieee80211_hw for the present interface but
with recent changes introduced the common->hw was never
updated and is required for virtual wiphys.

Cc: Jouni.Malinen <Jouni.Malinen@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>