fix cppcheck warning:

sound/soc/intel/atom/sst-mfld-platform-pcm.c:387:9: warning: Identical
condition and return expression 'ret_val', return value is always 0
[identicalConditionAfterEarlyExit]
 return ret_val;
        ^
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200813200147.61990-4-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>