net: dsa: use dsa_tree_for_each_user_port in dsa_slave_change_mtu

Use the more conventional iterator over user ports instead of explicitly
ignoring them, and use the more conventional name "other_dp" instead of
"dp_iter", for readability.

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