Fix cppcheck warning:

sound/soc/intel/atom/sst-mfld-platform-compress.c:46:14: style:
Variable 'ret_val' is assigned a value that is never
used. [unreadVariable]
 int ret_val = 0;
             ^
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200813200147.61990-3-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>