ASoC: SOF: debug: No need to export the snd_sof_debugfs_io_item()

The snd_sof_debugfs_io_item() only used within debug.c, no need to export
it.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Daniel Baluta <daniel.baluta@gmail.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Link: https://lore.kernel.org/r/20210915122116.18317-13-peter.ujfalusi@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>