ASoC: SOF: only allocate debugfs cache buffers for IPC flood entries

ASoC: SOF: only allocate debugfs cache buffers for IPC flood entries

snd_sof_debugfs_buf_item() is an exported function and is called from
different locations to initialise different debugfs entries. However
.cache_buf is only needed for IPC flood entries. Limit allocations
respectively.

Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Xiuli Pan <xiulipan@outlook.com>
Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210315163932.18663-3-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>