wifi: mac80211: fix link sta hash table handling

There are two issues here: we unhash the link stations only
directly before freeing the station they belong to, and we
also don't unhash all the links correctly in all cases. Fix
these issues.

Fixes: ba6ddab9 ("wifi: mac80211: maintain link-sta hash table")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>