ieee802154: 6lowpan: remove check on null

This patch removes one check on null which should be already done by
checking before for ARPHRD_IEEE802154. All ARPHRD_IEEE802154 and
ARPHRD_IEEE802154_MONITOR should have wdev->ieee802154_ptr, where
ARPHRD_IEEE802154 is currently a node interface only.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>