ASoC: SOF: sof-audio: skip prepare/unprepare if swidget is NULL

Skip preparing/unpreparing widgets if the swidget pointer is NULL. This
will be true in the case of virtual widgets in topology that were added
for reusing the legacy HDA machine driver with SOF.

Fixes: 9862dcf7

 ("ASoC: SOF: don't unprepare widget used other pipelines")
Cc: <stable@vger.kernel.org> # 6.1
Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Rander Wang <rander.wang@intel.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://lore.kernel.org/r/20230118101255.29139-3-peter.ujfalusi@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>