wifi: mac80211_hwsim: Don't access vif valid links directly

And instead use the vif getter functions, as a preparation for
supporting disabled/dormant links.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230608163202.8966bd5ce7c8.Ia73e3555aaf4ddf9917bced8d413fad08cc28f1b@changeid


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