wifi: mac80211: add ieee80211_vif_link_active() helper

wifi: mac80211: add ieee80211_vif_link_active() helper

We sometimes need to check if a link is active, and this
is complicated by the fact that active_links has no bits
set when the vif isn't (acting as) an MLD. Add a small
new helper ieee80211_vif_link_active() to make that a bit
easier, and use it in a few places.

Reviewed-by: Ilan Peer <ilan.peer@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240228094901.688760aff5f7.I06892a503f5ecb9563fbd678d35d08daf7a044b0@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>