mlxsw: spectrum: Sanitize bridge's upper devices

We're going to allow bridges stacked on top of port netdevs to be
enslaved to a VRF, but for now, only VLAN uppers of the VLAN-aware
bridge are supported.

Sanitize any other bridge upper. This is consistent with the way we
sanitize port netdevs' uppers.

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>