wifi: mac80211: fix NULL pointer deref with non-MLD STA

If we have a non-MLD STA on an AP MLD, we crash while
adding the station. Fix that, in this case we need to
use the STA's address also on the link data structure.

Fixes: f36fe0a2 ("wifi: mac80211: fix up link station creation/insertion")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>