mac80254: Provide real PAN coordinator info in beacons

Sending a beacon is a way to advertise a PAN, but also ourselves as
coordinator in the PAN. There is only one PAN coordinator in a PAN, this
is the device without parent (not associated itself to an "upper"
coordinator). Instead of blindly saying that we are the PAN coordinator,
let's actually use our internal information to fill this field.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Stefan Schmidt <stefan@datenfreihafen.org>
Acked-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/linux-wpan/20231128111655.507479-2-miquel.raynal@bootlin.com