sb1250: use netdev_alloc_skb

 Use netdev_alloc_skb.  This sets skb->dev and allows arch specific
allocation.  Also simplify and cleanup the alignment code.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>