mac80211: track assigned vifs in chanctx

This can be useful. Provides a more straghtforward
way to iterate over interfaces bound to a given
chanctx and allows tracking chanctx usage
explicitly.

The structure is protected by local->chanctx_mtx.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>