sh_eth: Really fix padding of short frames on TX

My previous fix to clear padding of short frames used skb->len as the
DMA length, assuming that skb_padto() extended skb->len to include the
padding.  That isn't the case; we need to use skb_put_padto() instead.

(This wasn't immediately obvious because software padding isn't
actually needed on the R-Car H2.  We could make it conditional on
which chip is being driven, but it's probably not worth the effort.)

Reported-by: "Violeta Menéndez González" <violeta.menendez@codethink.co.uk>
Fixes: 612a17a54b50 ("sh_eth: Fix padding of short frames on TX")
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>