ASoC: soc-dapm.c: add comment for kzalloc()/kfree() on snd_soc_dai_link_event_pre_pmu()

snd_soc_dai_link_event_pre_pmu() is using kzalloc()/kfree() for params.
It looks we don't need to use these, but are necessary.
But, it is difficult to know why it is necessary without any comments.
This patch adds the reasons via comment.

Link: https://lore.kernel.org/all/Yxc2wzbZsSVZNf8Y@sirena.org.uk/


Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/878rmubn47.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Mark Brown <broonie@kernel.org>