net: dsa: mv88e6xxx: Fix opps when adding vlan bridge

net: dsa: mv88e6xxx: Fix opps when adding vlan bridge

A port is not necessarily assigned to a netdev. And a port does not
need to be a member of a bridge. So when iterating over all ports,
check before using the netdev and bridge_dev for a port. Otherwise we
dereference a NULL pointer.

Fixes: da9c359e19f0 ("net: dsa: mv88e6xxx: check hardware VLAN in use")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>