tools: ynl: ethtool.py: Make tool invokable from any CWD

ethtool.py depends on yml files in a specific location of the linux kernel
tree. Using relative lookup for those files means that ethtool.py would
need to be run under tools/net/ynl/. Lookup needed yml files without
depending on the current working directory that ethtool.py is invoked from.

Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Link: https://lore.kernel.org/r/20240402204000.115081-1-rrameshbabu@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>