mlxsw: spectrum_router: Align RIF index allocation with existing code

The way we usually allocate an index is by letting the allocation
function return an error instead of an invalid index.

Do the same for RIF index.

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