mlxsw: spectrum: Return NOTIFY_BAD on bridge failure

It is possible for us to fail when joining or leaving a bridge, so let
the user know about that by returning NOTIFY_BAD, as already done for
LAG join/leave and 802.1D bridges.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>