mlxsw: spectrum_router: Pass nexthop netdev to mlxsw_sp_nexthop6_type_init()

Instead of passing the route and resolving the nexthop netdev from it,
pass the nexthop netdev directly.

This will later allow us to consolidate code paths between IPv4 and IPv6
code.

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