net: dsa: Add setter for SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS

In preparation for removing SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS_SUPPORT,
add support for a function that processes the
SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS and
SWITCHDEV_ATTR_ID_PORT_PRE_BRIDGE_FLAGS attributes and returns not
supported for any flag set, since DSA does not currently support
toggling those bridge port attributes (yet).

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