wifi: mac80211: Add getter functions for vif MLD state

As a preparation to support disabled/dormant links, add the
following function:

- ieee80211_vif_usable_links(): returns the bitmap of the links
  that can be activated. Use this function in all the places that
  the bitmap of the usable links is needed.

- ieee80211_vif_is_mld(): returns true iff the vif is an MLD.
  Use this function in all the places where an indication that the
  connection is a MLD is needed.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230608163202.86e3351da1fc.If6fe3a339fda2019f13f57ff768ecffb711b710a@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>