ASoC: dmaengine: do not use a NULL prepare_slave_config() callback

Even if struct snd_dmaengine_pcm_config is used, prepare_slave_config()
callback might not be set. Check if this callback is set before using it.

Fixes: fa654e08 ("ASoC: dmaengine-pcm: Provide default config")
Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Link: https://lore.kernel.org/r/20220307122202.2251639-2-codrin.ciubotariu@microchip.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/soc-generic-dmaengine-pcm.c

@@ -86,10 +86,10 @@ static int dmaengine_pcm_hw_params(struct snd_soc_component *component,
 
 	memset(&slave_config, 0, sizeof(slave_config));
 
-	if (!pcm->config)
-		prepare_slave_config = snd_dmaengine_pcm_prepare_slave_config;
-	else
+	if (pcm->config && pcm->config->prepare_slave_config)
 		prepare_slave_config = pcm->config->prepare_slave_config;
+	else
+		prepare_slave_config = snd_dmaengine_pcm_prepare_slave_config;
 
 	if (prepare_slave_config) {
 		int ret = prepare_slave_config(substream, params, &slave_config);