myri10ge: use eth_skb_pad helper

Update myri10ge to use eth_skb_pad helper.  This also corrects a minor
issue as the driver was updating length without updating the tail pointer.

Cc: Hyong-Youb Kim <hykim@myri.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>