ath9k: Remove ath_rate_softc

Move the hw rate tables to ath_softc, and access them directly.
tx_triglevel_max is global, move it to ath_rate_node.
Now that ath_rate_softc is gone, rate control attach becomes simpler.

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