Enhance the ethtool cable test interface by introducing the ability to
specify the source of the diagnostic information for cable test results.
This is particularly useful for PHYs that offer multiple diagnostic
methods, such as Time Domain Reflectometry (TDR) and Active Link Cable
Diagnostic (ALCD).

Key changes:
- Added `ethnl_cable_test_result_with_src` and
  `ethnl_cable_test_fault_length_with_src` functions to allow specifying
  the information source when reporting cable test results.
- Updated existing `ethnl_cable_test_result` and
  `ethnl_cable_test_fault_length` functions to use TDR as the default
  source, ensuring backward compatibility.
- Modified the UAPI to support these new attributes, enabling drivers to
  provide more detailed diagnostic information.
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20240822120703.1393130-3-o.rempel@pengutronix.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>