net: dsa: mv88e6xxx: check hardware VLAN in use

The DSA drivers now have access to the VLAN prepare phase and the bridge
net_device. It is easier to check for overlapping bridges from within
the driver. Thus add such check in mv88e6xxx_port_vlan_prepare.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>