net: dsa: microchip: ksz8795: Fix VLAN filtering

Currently ksz8_port_vlan_filtering() sets or clears the VLAN Enable
hardware flag.  That controls discarding of packets with a VID that
has not been enabled for any port on the switch.

Since it is a global flag, set the dsa_switch::vlan_filtering_is_global
flag so that the DSA core understands this can't be controlled per
port.

When VLAN filtering is enabled, the switch should also discard packets
with a VID that's not enabled on the ingress port.  Set or clear each
external port's VLAN Ingress Filter flag in ksz8_port_vlan_filtering()
to make that happen.

Fixes: e66f840c ("net: dsa: ksz: Add Microchip KSZ8795 DSA driver")
Signed-off-by: Ben Hutchings <ben.hutchings@mind.be>
Signed-off-by: David S. Miller <davem@davemloft.net>