iwlwifi: mvm: wait for stop sched-scan completion

cfg80211 assumes a scheduled scan is stopped synchronously. Wait for the
FW before returning to caller.

Don't do anything in the async handler in the stop-from-above flow.
There's no need to call the mac80211 sched-scan completion as the
cleanup will be automatic. Make sure the async handler is called before
the next incoming scan changes the scan_status by flushing the async
handlers after all invocations.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>