mlxsw: spectrum_router: Allow appending to dev-only routes

Commit f34436a4 ("net/ipv6: Simplify route replace and appending
into multipath route") changed the IPv6 route append logic so that
dev-only routes can be appended and not only gatewayed routes.

Align mlxsw with the new behaviour.

Fixes: f34436a4

 ("net/ipv6: Simplify route replace and appending into multipath route")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>