ASoC: simple-card-utils: accept NULL parameter on asoc_simple_card_xxx()

If simple-card-utils accept NULL pointer on asoc_simple_card_xxx(),
each driver code will be more simple.
Let's accept NULL pointer.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/generic/simple-card-utils.c

@@ -153,13 +153,17 @@ EXPORT_SYMBOL_GPL(asoc_simple_card_parse_card_name);
 
 int asoc_simple_card_clk_enable(struct asoc_simple_dai *dai)
 {
-	return clk_prepare_enable(dai->clk);
+	if (dai)
+		return clk_prepare_enable(dai->clk);
+
+	return 0;
 }
 EXPORT_SYMBOL_GPL(asoc_simple_card_clk_enable);
 
 void asoc_simple_card_clk_disable(struct asoc_simple_dai *dai)
 {
-	clk_disable_unprepare(dai->clk);
+	if (dai)
+		clk_disable_unprepare(dai->clk);
 }
 EXPORT_SYMBOL_GPL(asoc_simple_card_clk_disable);
 
@@ -342,6 +346,9 @@ int asoc_simple_card_init_dai(struct snd_soc_dai *dai,
 {
 	int ret;
 
+	if (!simple_dai)
+		return 0;
+
 	if (simple_dai->sysclk) {
 		ret = snd_soc_dai_set_sysclk(dai, 0, simple_dai->sysclk,
 					     simple_dai->clk_direction);