tools: ynl: Rename ethtool to ethtool.py

Make it explicit that this tool is not a drop-in replacement for ethtool.
This tool is intended for testing ethtool functionality implemented in the
kernel and should use a name that differentiates it from the ethtool
utility.

Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Link: https://lore.kernel.org/r/20230413012252.184434-2-rrameshbabu@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>