nm_platform_ethtool_init_ring() only has one caller. It's simpler to
drop the function and implement it at the only place where it is needed.

Maybe there could be a place for a function to initialize NMEthtoolRingState,
one option after the other. However, at the moment there is only one
user, so don't implement it.

This fixes various minor issues:

- the function had a NMPlatform argument, although the argument
  is not used. Thus function merely operates on a NMEthtoolRingState
  instance and shouldn't have a nm_platform_*() name.

- nm_platform_ethtool_init_ring() returned a boolean, but all
  code paths (except assertion failures) returned success.

- as the function returned an error status, the caller was compelled
  to handle an error that could never happen.

- the option was specified by name, although we already have a more
  efficient way to express the option: the NMEthtoolID. Also, the
  caller already needed to resolve the name to the NMEthtoolID, so
  there was no need to again lookup the ID by name.