wifi: mac80211: mlme: first adjustments for MLO

Do the first adjustments in the client-side code to pass
the link pointer (instead of sdata) to most places etc.

This is just preparation, so the real MLO patches become
smaller.

Note that this isn't complete, notably there are still
quite a few references to sta->deflink and sta->sta.deflink.

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