dbus: omit ethernet speed,duplex,autoneg defaults

As with applied pull #452 we do not drop the complete ethernet struct any more when the driver does not support GSET but e.g. offload only, show-xml started to show useless defaults like:
```
  <ethernet>
    <link-speed>0</link-speed>
    <port-type>default</port-type>
    <duplex>full</duplex>
    <autoneg-enable>default</autoneg-enable>
    ...
  </ethernet>
```
This commit changes to use custom get/set functions for them to omit these useless defaults.