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>