ath9k: Cache BSS information

Using the BSS information stored in mac80211 directly
is racy in certain conditions. For example, in a MCC
setup, if the scheduler is switching channels when
a local deauth is issued, calculation of the opmode/bssid
etc. is incorrect. To avoid this, store the bss params
in the driver and use it.

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