ASoC: audio-graph-card: tidyup return method from probe()

Current return method from probe() is very confusable.
This patch tidyup it to normal return method

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>