ath9k: Add support for multiple secondary virtual wiphys

The new struct ath_softc::sec_wiphy array is used to store information
about virtual wiphys and select which wiphy is used in calls to
mac80211. Each virtual wiphy will be assigned a different MAC address
based on the virtual wiphy index.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>