mac802154: Avoid new associations while disassociating

While disassociating from a PAN ourselves, let's set the maximum number
of associations temporarily to zero to be sure no new device tries to
associate with us.

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/20231128111655.507479-6-miquel.raynal@bootlin.com