network_diagnostics: add output of ethtool for wired links

Add output of ethtool to network_diagnostics to help with debugging
issues with wired ethernet connections.

BUG=chromium:383508
TEST=tested on wolf with base image

Change-Id: I5a435e2aa5d200cd44f1ee5fa882d21f0ba94c27
Reviewed-on: https://chromium-review.googlesource.com/207084
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Samuel Tan <samueltan@chromium.org>
Commit-Queue: Samuel Tan <samueltan@chromium.org>
1 file changed