tools: ynl: Remove absolute paths to yaml files from ethtool testing tool

tools: ynl: Remove absolute paths to yaml files from ethtool testing tool

Absolute paths for the spec and schema files make the ethtool testing tool
unusable with freshly checked-out source trees. Replace absolute paths with
relative paths for both files in the Documentation/ directory.

Issue seen before the change

  Traceback (most recent call last):
    File "/home/binary-eater/Documents/mlx/linux/tools/net/ynl/./ethtool", line 424, in <module>
      main()
    File "/home/binary-eater/Documents/mlx/linux/tools/net/ynl/./ethtool", line 158, in main
      ynl = YnlFamily(spec, schema)
    File "/home/binary-eater/Documents/mlx/linux/tools/net/ynl/lib/ynl.py", line 342, in __init__
      super().__init__(def_path, schema)
    File "/home/binary-eater/Documents/mlx/linux/tools/net/ynl/lib/nlspec.py", line 333, in __init__
      with open(spec_path, "r") as stream:
  FileNotFoundError: [Errno 2] No such file or directory: '/usr/local/google/home/sdf/src/linux/Documentation/netlink/specs/ethtool.yaml'

Fixes: f3d07b02b2b8 ("tools: ynl: ethtool testing tool")
Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Link: https://lore.kernel.org/r/20230413012252.184434-1-rrameshbabu@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>