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>