net: dsa: drop dsa_slave_priv from dsa_slave_change_mtu

We can get a hold of the "ds" pointer directly from "dp", no need for
the dsa_slave_priv.

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