net: ethtool: Add impedance mismatch result code to cable test

Some PHYs can recognize during a cable test if the impedance in the cable
is okay. They can detect reflections caused by impedance discontinuity
between a regular 100 Ohm cable and an abnormal part with a higher or
lower impedance.

This commit introduces a new result code:
ETHTOOL_A_CABLE_RESULT_CODE_IMPEDANCE_MISMATCH,
which represents the results of a cable test indicating issues with
impedance integrity.
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240402201123.2961909-2-paweldembicki@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/ethtool_netlink.h

@@ -515,6 +515,10 @@ enum {
 	ETHTOOL_A_CABLE_RESULT_CODE_OPEN,
 	ETHTOOL_A_CABLE_RESULT_CODE_SAME_SHORT,
 	ETHTOOL_A_CABLE_RESULT_CODE_CROSS_SHORT,
+	/* detected reflection caused by the impedance discontinuity between
+	 * a regular 100 Ohm cable and a part with the abnormal impedance value
+	 */
+	ETHTOOL_A_CABLE_RESULT_CODE_IMPEDANCE_MISMATCH,
 };
 
 enum {