mac80211: stop scan before connection

Stop scan before authentication or association to make sure
that nothing interferes with connection flow.

Currently mac80211 defers RX auth and assoc packets (among other ones)
until after the scan is complete, so auth during scan is likely to fail
if scan took too much time.

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Reviewed-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>