mlxsw: spectrum_router: Use helper to check for last neighbor

Use list_is_last helper to check for last neighbor.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>