ath9k: Revamp wireless mode usage

Use a single enum for managing modes, store supported modes by
the HW in a bitmask.
Register legacy rates with mac80211 only at init.

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