ASoC: SOF: PM: add check before setting d0_substate

Add check before seeting d0_substate and return success if Audio DSP is
already in the target substate.
Signed-off-by: Keyon Jie <yang.jie@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191111223343.19986-3-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/pm.c

@@ -416,6 +416,9 @@ int snd_sof_set_d0_substate(struct snd_sof_dev *sdev,
 {
 	int ret;
 
+	if (sdev->d0_substate == d0_substate)
+		return 0;
+
 	/* do platform specific set_state */
 	ret = snd_sof_dsp_set_power_state(sdev, d0_substate);
 	if (ret < 0)