net: dsa: avoid one dsa_to_port() in dsa_slave_change_mtu

We could retrieve the cpu_dp pointer directly from the "dp" we already
have, no need to resort to dsa_to_port(ds, port).

This change also removes the need for an "int port", so that is also
deleted.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>