wifi: mac80211: add ieee80211_tdls_sta_link_id()

We've open-coded this twice and will need it again,
add ieee80211_tdls_sta_link_id() to get the one link
ID for a TDLS STA.

Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20240612143707.9f8141ae1725.I343822bbba0ae08dedb2f54a0ce87f2ae5ebeb2b@changeid


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