ieee802154: 6lowpan: fix return of netdev notifier

This patch fixed the return value of netdev notifier. If the command is
a don't care a NOTIFY_DONE should be returned. If the command matched a
NOTIFY_OK should be returned.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>