net: mscc: ocelot: remove unnecessary stat reading from ethtool

The ocelot_update_stats function only needs to read from one port, yet it
was updating the stats for all ports. Update to only read the stats that
are necessary.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>