mlxsw: spectrum_router: Store weight in nexthop struct

As the first step towards non-equal-cost multi-path support, store each
nexthop's weight.

For IPv6 nexthops always set the weight to 1, as it only supports ECMP.

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>