ASoC: fsl-ssi: Fix stats compile warning

single_open requires a function with signature
'int (*)(struct seq_file *, void *)'. This patch fixes the warning by
fixing the wrong return type of fsl_ssi_stats_show.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>