wifi: iwlwifi: mvm: clean up mac_id vs. link_id in MLD sta

Here we always have a link ID, not MAC ID, so clean up the
naming.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230329100040.3def62de34b5.I10c9cf5dbfd1fc1e9c9c7d6d4cefcf0c08f1f2da@changeid


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