mlxsw: Add ECN configurations with IPinIP tunnels

Initialize ECN mapping registers during router init according to
INET_ECN_encapsulate() and INET_ECN_decapsulate().

For IP-in-IP encapsulation, this is required to ensure that ECN bits in
the underlay are set in accordance with the kernel. For decapsulation,
this is required to ensure that packets with invalid ECN combination in
underlay and overlay are trapped to the kernel and not forwarded.

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>