tools/net/ynl: ethtool: support spec load from install location

Replace hard-coded paths for spec and schema with lookup functions so
that ethtool.py will work in-tree or when installed.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250111154803.7496-2-donald.hunter@gmail.com


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