ieee802154: Add support for limiting the number of associated devices

Coordinators may refuse associations. We need a user input for
that. Let's add a new netlink command which can provide a maximum number
of devices we accept to associate with as a first step. Later, we could
also forward the request to userspace and check whether the association
should be accepted or not.

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/20230927181214.129346-9-miquel.raynal@bootlin.com