ath9k: configure bss info at assoc notification

The proper place to configure bss info is at assoc notification. So that
ath9k continues to work if the supirous bssid notification will be
removed in future.

Cc: Johannes Berg <johannes@sipsolutions.net>
Reported-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>