ieee802154: Avoid confusing changes after associating

Once associated with any device, we are part of a PAN (with a specific
PAN ID), and we are expected to be present on a particular
channel. Let's avoid confusing other devices by preventing any PAN
ID/channel change once associated.

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/20231128111655.507479-5-miquel.raynal@bootlin.com