net: bridge: Stop calling switchdev_port_attr_get()

Now that all switchdev drivers have been converted to check the
SWITCHDEV_ATTR_ID_PORT_PRE_BRIDGE_FLAGS flags and report flags that they
do not support accordingly, we can migrate the bridge code to try to set
that attribute first, check the results and then do the actual setting.

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