sc92031: start transmit return value bugfix

Any negative return value from start_xmit is interpreted as NETDEV_TX_LOCK
which is not what this driver wants. It should return 0 (NETDEV_TX_OK)
when it consumes a packet.

Also, use skb_padto() as the generic way to pad small frames.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>