iwlwifi: don't delete AP station directly

With the mac80211 deauth sequence changes, the
station is deleted before the device is set
unassociated. This can cause the device to get
confused as it expects the station to be there
while the associated bit is set.

To fix this, do not delete the AP station from
the device when mac80211 asks for deletion,
instead just mark it as unused and rely on the
unassociated RXON to drop it from the station
database in the device.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>