ASoC: rsnd: use ring buffer for rsnd_mod_name()

commit c0ea089d ("ASoC: rsnd: rsnd_mod_name() handles both name and
ID") merged "name" and "ID" on rsnd_mod_name() to handle sub-ID
(= for CTU/BUSIF).
Then, it decided to share static char to avoid pointless memory.
But, it doesn't work correctry in below case, because last called
name will be used.

	dev_xxx(dev, "%s is connected to %s\n",
		rsnd_mod_name(mod_a),  /* ssiu[00] */
		rsnd_mod_name(mod_b)); /* ssi[0]   */
	->
	rcar_sound ec500000.sound: ssi[0] is connected to ssi[0]
	                           ~~~~~~                 ~~~~~~
We still don't want to have pointless memory, so let's use ring buffer.
16byte x 5 is very enough for this purpose.

	dev_xxx(dev, "%s is connected to %s\n",
		rsnd_mod_name(mod_a),  /* ssiu[00] */
		rsnd_mod_name(mod_b)); /* ssi[0]   */
	->
	rcar_sound ec500000.sound: ssiu[00] is connected to ssi[0]
	                           ~~~~~~~~                 ~~~~~~
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sh/rcar/core.c

@@ -137,10 +137,17 @@ struct dma_chan *rsnd_mod_dma_req(struct rsnd_dai_stream *io,
 	return mod->ops->dma_req(io, mod);
 }
 
+#define MOD_NAME_NUM   5
 #define MOD_NAME_SIZE 16
 char *rsnd_mod_name(struct rsnd_mod *mod)
 {
-	static char name[MOD_NAME_SIZE];
+	static char names[MOD_NAME_NUM][MOD_NAME_SIZE];
+	static int num;
+	char *name = names[num];
+
+	num++;
+	if (num >= MOD_NAME_NUM)
+		num = 0;
 
 	/*
 	 * Let's use same char to avoid pointlessness memory