selftests/alsa: make dump_config_tree() as void function

dump_config_tree() is declared to return an int, but the compiler cannot
prove that it always returns any value at all. This leads to a clang
warning, when building via:

    make LLVM=1 -C tools/testing/selftests

Suggested-by: John Hubbard <jhubbard@nvidia.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20240506075419.301780-1-perex@perex.cz