6lowpan: use netdev_alloc_skb instead dev_alloc_skb

This patch uses the netdev_alloc_skb instead dev_alloc_skb function and
drops the seperate assignment to 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>