net: systemport: Utilize skb_put_padto()

Since we need to pad our packets, utilize skb_put_padto() which
increases skb->len by how much we need to pad, allowing us to eliminate
the test on skb->len right below.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>