net: dsa: Add support for querying supported bridge flags

The DSA drivers do not support bridge flags offload. Yet, this attribute
should be added in order for the bridge to fail when one tries set a
flag on the port, as explained in commit dc0ecabd ("net: switchdev:
Add support for querying supported bridge flags by hardware").

Signed-off-by: Arkadi Sharshevsky <arkadis@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>