net: ethernet: ti: cpsw: remove netif_trans_update

No need to update jiffies in txq->trans_start twice, it's supposed to be
done in netdev_start_xmit() and anyway is re-written. Also, no reason to
update trans time in case of an error.

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