ath9k: Fix panic when adding an AP interface

ath9k: Fix panic when adding an AP interface

If a station interface is already assigned to a context
and is active and a second interface of type AP is added,
then beaconing on the new interface has to be begin only
after the BSS_CHANGED_BEACON_ENABLED flag is sent by mac80211
to the driver.

But, since we issue ATH_CHANCTX_EVENT_ENABLE_MULTICHANNEL as soon
as a new channel context is added, a switch occurs almost immediately
before BSS_CHANGED_BEACON_ENABLED is received. When a HW reset
is done for the new context, beacons are enabled for the
interface since "enable_beacon" in the BSS config maintained
in mac80211 is true - but the driver hasn't been notified yet.
This causes a panic, since the beacon interval is zero for this
interface and ath9k_cmn_beacon_config_ap() doesn't have a safety check.

Fix this panic by checking if the beacon params has been cached
for this context and use the "enable_beacon" flag maintained
locally in the driver. Also, recalculate the summary data
after the beacon params have been cached when BSS_CHANGED_BEACON_ENABLED
is received.

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