ath9k: Use rate_driver_data

Remove the hack using vif, and use rate_driver_data within
skb->cb to hold driver specific rate information.
Setup the rate series in the skb's tx control area and remove
all references to ath9k specific rate series ( using struct ath_rc_series ).

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