wifi: nl80211/mac80211: clarify link ID in control port TX

Clarify the link ID behaviour in control port TX, we need it
to select the link to transmit on for both MLD and non-MLD
receivers, but select the link address as the SA only if the
receiver is not an MLD.

Fixes: 67207bab

 ("wifi: cfg80211/mac80211: Support control port TX from specific link")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>