net: dsa: felix: use DSA port iteration helpers

Use the helpers that avoid the quadratic complexity associated with
calling dsa_to_port() indirectly: dsa_is_unused_port(),
dsa_is_cpu_port().

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