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>