tools: ynl-gen-c: annotate valid choices for --mode

This makes argparse validate the input and helps users
understand which modes are possible.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20241206113100.e2ab5cf6937c.Ie149a0ca5df713860964b44fe9d9ae547f2e1553@changeid
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/ynl-gen-c.py

@@ -2706,7 +2706,8 @@ def find_kernel_root(full_path):
 
 def main():
     parser = argparse.ArgumentParser(description='Netlink simple parsing generator')
-    parser.add_argument('--mode', dest='mode', type=str, required=True)
+    parser.add_argument('--mode', dest='mode', type=str, required=True,
+                        choices=('user', 'kernel', 'uapi'))
     parser.add_argument('--spec', dest='spec', type=str, required=True)
     parser.add_argument('--header', dest='header', action='store_true', default=None)
     parser.add_argument('--source', dest='header', action='store_false')