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

tools/testing/selftests/alsa/conf.c

@@ -105,7 +105,7 @@ static struct card_cfg_data *conf_data_by_card(int card, bool msg)
 	return NULL;
 }
 
-static int dump_config_tree(snd_config_t *top)
+static void dump_config_tree(snd_config_t *top)
 {
 	snd_output_t *out;
 	int err;