net: ethernet: ti: davinci_emac: switch to use skb_put_padto()

Use skb_put_padto() instead of skb_padto() so skb->len also got updated, as
preparation for further removing frame padding from cpdma.
It also makes xmit path more clear and linear.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>