wifi: mac80211: add ieee80211_find_sta_by_link_addrs API

Add a new API function ieee80211_find_sta_by_link_addrs()
that looks up the STA and link ID based on interface and
station link addresses.

We're going to use it for mac80211-hwsim to track on the
AP side which links are active.

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