ieee802154: Let PAN IDs be reset

Soon association and disassociation will be implemented, which will
require to be able to either change the PAN ID from 0xFFFF to a real
value when association succeeded, or to reset the PAN ID to 0xFFFF upon
disassociation. Let's allow to do that manually for now.

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/20230927181214.129346-2-miquel.raynal@bootlin.com