ath9k: Cleanup multiple VIF processing

Replace the internal sc_vaps array and index values by using vif
pointer from mac80211. Allow multiple VIPs to be registered. Though,
number of beaconing VIFs is still limited by ATH_BCBUF (currently
1). Multiple virtual STAs support is not yet complete, but at least
the data structures should now be able to handle this.

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