net: dsa: Deny enslaving VLAN devices into VLAN aware bridge

VLAN devices on top of a DSA network device which is already part of a
bridge and with said bridge being VLAN aware should not be allowed to be
enslaved into that bridge. For one, this duplicates functionality
offered by the VLAN aware bridge which supports tagged and untagged VLAN
frames processing and it would make things needlessly complex to e.g.:
propagate FDB/MDB accordingly.

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