ASoC: SOF: remove superfluous NULL check in debugfs read

ASoC: SOF: remove superfluous NULL check in debugfs read

When reading from IPC flood debugfs entries no need to check whether
.cache_buf is NULL - it's impossible since otherwise the initialisation
would have failed. This also fixes a klocwork reported issue:

passed to function and may be dereferenced there by passing argument 2
to function 'memcpy' at line 510.
sound/soc/sof/debug.c:510 | sof_dfsentry_read()

Reported-by: Keqiao Zhang <keqiao.zhang@intel.com>
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/20210315163932.18663-4-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>