wifi: mac80211: fix up link station creation/insertion

When we create a station with a non-default link, then
we should have a link address, and we definitely need
to insert it into the link hash table on insertion.

Split the API into with and without link creation and
if it has a link, insert the link into the link hash
table on sta_info_insert().

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