mac80211: allow iterating inactive interfaces

Sometimes the driver might want to modify private data in interfaces
that are down. One possible use-case is cleaning up interface state
after HW recovery. Some interfaces that were up before the recovery took
place might be down now, but they might still be "dirty".

Introduce a new iterate_interfaces() API and a new ACTIVE iterator flag.
This way the internal implementation of the both active and inactive
APIs remains the same.

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