Imagine you have a veth device. That device supports certain offload features
(like "ethtool.feature-rx-checksum") but doesn't support any ring
options. Even trying to read the current ring settings will fail.

If you try to activate that profile, NMDevice previously would always
try to fetch the ring options and log a warning and extra debugging
messages:

  <trace> [1590511552.3943] ethtool[31]: ETHTOOL_GRINGPARAM, v: failed: Operation not supported
  <trace> [1590511552.3944] ethtool[31]: get-ring: failure getting ring settings
  <warn>  [1590511552.3944] device (v): ethtool: failure getting ring settings (cannot read)

It does so, although you didn't specify any ring settings and there
was no need to fetch the ring settings to begin with.

Avoid this extra logging by only fetching the ring option when they
are actually required.