6lowpan: remove skb->dev assignment

This patch removes the assignment of skb->dev. We don't need it here because
we use the netdev_alloc_skb_ip_align function which already sets the
skb->dev.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reviewed-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: David S. Miller <davem@davemloft.net>