ath9k: Do not use association state to update rate table

Now that HT information is made available to the driver through
config() callback, the channel type can be calculated whenever
setting the channel. Update the rate table accordingly with the new
channel type.

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