ASoC: SOF: sof-audio: remove useless assignment

ASoC: SOF: sof-audio: remove useless assignment

There is no need to assign spcm to NULL. Removing this assignment also
removes a false alarm reported by cppcheck.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Rander Wang <rander.wang@intel.com>
Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://lore.kernel.org/r/20220421162600.302230-1-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>