ethernet: handle ethtool offload as independent structure
ethtool: Refactor handling of independed ethtool ioctl

One ioctl failing should not block the other ones.
dbus: omit ethernet speed,duplex,autoneg defaults
Merge pull request #452 from pwieczorkiewicz/ethtool-nofail
Merge pull request #454 from mtomaschewski/ethtool-defaults