nl802154: avoid address change while running lowpan

The generation of autoconfigured IPv6 link-local addresses starts with a
notification on interface up.

To handle autoconfiguration according to RFC 4944 requires pan id and
short address to generate an autoconfigured link-local address. This
patch will avoid changing of these link-layer address configuration
while lowpan interface is up.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>