net: dsa: b53: Use vlan_filtering property from dsa_switch

While possible (and safe) to use the newly introduced
dsa_port_is_vlan_filtering helper, fabricating a dsa_port pointer is a
bit awkward, so simply retrieve this from the dsa_switch structure.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>