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