net: define an enum for the napi threaded state

Instead of using '0' and '1' for napi threaded state use an enum with
'disabled' and 'enabled' states.

Tested:
 ./tools/testing/selftests/net/nl_netdev.py
 TAP version 13
 1..7
 ok 1 nl_netdev.empty_check
 ok 2 nl_netdev.lo_check
 ok 3 nl_netdev.page_pool_check
 ok 4 nl_netdev.napi_list_check
 ok 5 nl_netdev.dev_set_threaded
 ok 6 nl_netdev.napi_set_threaded
 ok 7 nl_netdev.nsim_rxq_reset_down
 # Totals: pass:7 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Samiullah Khawaja <skhawaja@google.com>
Link: https://patch.msgid.link/20250723013031.2911384-4-skhawaja@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/netdev.yaml

@@ -85,6 +85,10 @@ definitions:
     name: qstats-scope
     type: flags
     entries: [queue]
+  -
+    name: napi-threaded
+    type: enum
+    entries: [disabled, enabled]
 
 attribute-sets:
   -
@@ -286,11 +290,10 @@ attribute-sets:
       -
         name: threaded
         doc: Whether the NAPI is configured to operate in threaded polling
-             mode. If this is set to 1 then the NAPI context operates in
-             threaded polling mode.
-        type: uint
-        checks:
-          max: 1
+             mode. If this is set to enabled then the NAPI context operates
+             in threaded polling mode.
+        type: u32
+        enum: napi-threaded
   -
     name: xsk-info
     attributes: []