ath9k: Fix bug in rate table management

The proper rate table wouldn't be used if a disassoc
happens and a new attempt is made to associate using
wpa_supplicant. This patch fixes it by storing the rate
table to be used on association.

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