mac802154: Only allow PAN controllers to process association requests

It is not very clear in the specification whether simple coordinators
are allowed or not to answer to association requests themselves. As
there is no synchronization mechanism, it is probably best to rely on
the relay feature of these coordinators and avoid processing them in
this case.

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/20231128111655.507479-4-miquel.raynal@bootlin.com