s390/qeth: don't bother updating the last-tx time

As the documentation for netif_trans_update() says, netdev_start_xmit()
already updates the last-tx time after every good xmit. So don't
duplicate that effort.

One odd case is that qeth_flush_buffers() also gets called from our
TX completion handler, to flush out any partially filled buffer when
we switch the queue to non-packing mode. But as the TX completion
handler will _always_ wake the txq, we don't have to worry about
the TX watchdog there.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>