net: dsa: sja1105: Clear VLAN filtering offload netdev feature

The switch barely supports traffic I/O, and it does that by repurposing
VLANs when there is no bridge that is taking control of them.

Letting DSA declare this netdev feature as supported (see
dsa_slave_create) would mean that VLAN sub-interfaces created on sja1105
switch ports will be hardware offloaded. That means that
net/8021q/vlan_core.c would install the VLAN into the filter tables of
the switch, potentially interfering with the tag_8021q VLANs.

We need to prevent that from happening and not let the 8021q core
offload VLANs to the switch hardware tables. In vlan_filtering=0 modes
of operation, the switch ports can pass through VLAN-tagged frames with
no problem.

Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>