ethtool: add linkmode bitmap support to struct ethtool_keee

Add linkmode bitmap members to struct ethtool_keee, but keep the legacy
u32 bitmaps for compatibility with existing drivers.
Use linkmode "supported" not being empty as indicator that a user wants
to use the linkmode bitmap members instead of the legacy bitmaps.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>