dsa: port: Ignore bridge VLAN events

A follow-up patch enables emitting VLAN notifications for the bridge CPU
port in addition to the existing slave port notifications. These
notifications have orig_dev set to the bridge in question.

Because there's no specific support for these VLANs, just ignore the
notifications to maintain the current behavior.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>