It might seem a bit pointless to do a multi-link operation
connection with just a single link, but this is already a
big change, so for now, limit MLO connections to a single
link.

Extending that to multiple links will require
 * work on parsing the multi-link element with STA profile
   properly, including element fragmentation;
 * checking the per-link status in the multi-link element
 * implementing logic to have active/inactive links to let
   drivers decide which links should be active;
 * implementing multicast RX deduplication;
 * and likely more.

For now this is still useful since it lets us do multi-link
connections for the purposes of testing APIs and the higher
layers such as wpa_supplicant.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>