net: switchdev: ignore unsupported bridge flags

switchdev_port_bridge_getlink() queries SWITCHDEV_ATTR_PORT_BRIDGE_FLAGS
attributes, but a driver doesn't need to implement this in order to get
bridge link information.

So error out only on errors different than -EOPNOTSUPP.

(This is a follow-up patch for 7d4f8d87.)

Fixes: 8793d0a6

 ("switchdev: add new switchdev_port_bridge_getlink")
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>