mlxsw: spectrum_router: Don't destroy RIF if L3 slave

We usually destroy the netdev's router interface (RIF) when the last IP
address is removed from it.

However, we shouldn't do that if it's enslaved to an L3 master device.

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>