selftests: net: mscc: ocelot: bring up the ports automatically

Looks like when I wrote the selftests I was using a network manager that
brought up the ports automatically. In order to not rely on that, let
the script open them up.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>