mlxsw: spectrum_router: Resolve RIF from nexthop struct instead of neighbour

The two are the same, but for blackhole nexthops we will not have an
associated neighbour struct, so resolve the RIF from the nexthop struct
itself instead.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>