mlxsw: spectrum_router: Enable IPv6 router

Before we add IPv6 constructs like traps and router interfaces, we first
need to enable IPv6 routing in the device.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
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>