tools: ynl: fix undefined variable name

tools: ynl: fix undefined variable name

This variable used in the error path was not defined according to Ruff.
msg_format.attr_set is used instead, presumably the one that was
supposed to be used originally.

This is linked to Ruff error F821 [1]:

  An undefined name is likely to raise NameError at runtime.

Fixes: 1769e2be4baa ("tools/net/ynl: Add 'sub-message' attribute decoding to ynl")
Link: https://docs.astral.sh/ruff/rules/undefined-name/
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Link: https://patch.msgid.link/20250909-net-next-ynl-ruff-v1-1-238c2bccdd99@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>