net: ethtool: tsconfig: Fix netlink type of hwtstamp flags

Fix the netlink type for hardware timestamp flags, which are represented
as a bitset of flags. Although only one flag is supported currently, the
correct netlink bitset type should be used instead of u32 to keep
consistency with other fields. Address this by adding a new named string
set description for the hwtstamp flag structure.

The code has been introduced in the current release so the uAPI change is
still okay.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Fixes: 6e9e2eed4f ("net: ethtool: Add support for tsconfig command to get/set hwtstamp config")
Link: https://patch.msgid.link/20250205110304.375086-1-kory.maincent@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/ethtool.h

@@ -682,6 +682,7 @@ enum ethtool_link_ext_substate_module {
  * @ETH_SS_STATS_ETH_CTRL: names of IEEE 802.3 MAC Control statistics
  * @ETH_SS_STATS_RMON: names of RMON statistics
  * @ETH_SS_STATS_PHY: names of PHY(dev) statistics
+ * @ETH_SS_TS_FLAGS: hardware timestamping flags
  *
  * @ETH_SS_COUNT: number of defined string sets
  */
@@ -708,6 +709,7 @@ enum ethtool_stringset {
 	ETH_SS_STATS_ETH_CTRL,
 	ETH_SS_STATS_RMON,
 	ETH_SS_STATS_PHY,
+	ETH_SS_TS_FLAGS,
 
 	/* add new constants above here */
 	ETH_SS_COUNT