net: ethernet: ti: cpsw: return NET_XMIT_DROP if skb_padto failed

If skb_padto failed the skb has been dropped already, so it was
consumed, but it doesn't mean it was sent, thus no need to update
queue tx time, etc. So, return NET_XMIT_DROP as more appropriate.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>