mlxsw: spectrum_ethtool: Add link extended state

Implement .get_down_ext_state() as part of ethtool_ops.
Query link down reason from PDDR register and convert it to ethtool
link_ext_state.

In case that more information than common link_ext_state is provided,
fill link_ext_substate also with the appropriate value.

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