ethtool: provide WoL settings with WOL_GET request

Implement WOL_GET request to get wake-on-lan settings for a device,
traditionally available via ETHTOOL_GWOL ioctl request.

As part of the implementation, provide symbolic names for wake-on-line
modes as ETH_SS_WOL_MODES string set.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>