net: dsa: fix dsa_port_vlan_filtering when global

The blamed refactoring commit changed a "port" iterator with "other_dp",
but still looked at the slave_dev of the dp outside the loop, instead of
other_dp->slave from the loop.

As a result, dsa_port_vlan_filtering() would not call
dsa_slave_manage_vlan_filtering() except for the port in cause, and not
for all switch ports as expected.

Fixes: d0004a02 ("net: dsa: remove the "dsa_to_port in a loop" antipattern from the core")
Reported-by: Lucian Banu <Lucian.Banu@westermo.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>