ASoC: fsl-ssi: Move sysfs stats to debugfs

Interrupt statistics of fsl_ssi are mainly for debugging purpose. Most
of those interrupts show error states, e.g. under/overflow.
The stats should be exposed via debugfs instead of sysfs.

This patch moves the statistics file to debugfs.

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