ASoC: soc-core: add comment for rtd freeing

ASoC: soc-core: add comment for rtd freeing

We don't need to mind freeing for rtd,
because it was created from devm_kzalloc(dev, xxx) which is rtd->dev.

This means, if rtd->dev was freed, rtd will be also freed
automatically.

	soc_new_pcm_runtime(...)
	{
		...
		rtd = devm_kzalloc(dev, ...);
		rtd->dev = dev;
		...
	}

This explanation was missing at soc_free_pcm_runtime() comment.
This patch indicates it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87sg4dxldn.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>