ASoC: soc-dai.h: don't use discriminatory terms for comment

soc-dai is using discriminatory terms for comment.
This patch renames "slave" to "secondary", thus
we can keep M/S initials.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/875zam3bmk.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>