mlxsw: spectrum_router: Don't batch neighbour deletion

Current firmware supported by the driver doesn't support batch deletion
of IPv6 neighbours on a given router interface (RIF).

Until a new version that supports this functionality is made available,
delete neighbours one by one.

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>