mac802154: Follow the number of associated devices

Track the count of associated devices. Limit the number of associations
using the value provided by the user if any. If we reach the maximum
number of associations, we tell the device we are at capacity. If the
user do not want to accept any more associations, it may specify the
value 0 to the maximum number of associations, which will lead to an
access denied error status returned to the peers trying to associate.

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/20230927181214.129346-10-miquel.raynal@bootlin.com