wifi: iwlwifi: mvm: 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/20230615094410.61ca688cbbf1.Ic1b4049cf156238ff16e6c57959004da911cb5c8@changeid


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