staging: octeon: Add a workaround for the issue of GCC8

In function cvm_oct_xmit(), the packet length is set with code like
this:
  pko_command.s.total_bytes = skb->len;

But when building with gcc8, the pko_command.s.total_bytes doesn't
emit the right value when it is used in the following codes:
        /* Send the packet to the output queue */
        if (unlikely(cvmx_pko_send_packet_finish(priv->port,
                                                 priv->queue + qos,
                                                 pko_command, hw_buffer,
                                                 CVMX_PKO_LOCK_NONE))) {

Adding a barrier after the assignment would generate the right codes.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>