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>

Documentation/netlink/specs/ethtool.yaml

@@ -1524,7 +1524,8 @@ attribute-sets:
         nested-attributes: bitset
       -
         name: hwtstamp-flags
-        type: u32
+        type: nest
+        nested-attributes: bitset
 
 operations:
   enum-model: directional