mlxsw: spectrum_router: Consolidate nexthop helpers

The helper mlxsw_sp_nexthop_offload() is actually interested in finding
out if the nexthop is both written to the adjacency table and forwarding
packets (as opposed to discarding them).

Rename it to mlxsw_sp_nexthop_is_forward() and remove
mlxsw_sp_nexthop_is_discard().

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