UPSTREAM: ALSA: doc: Clarify IEC958 controls iface

The doc currently mentions that the IEC958 Playback Default should be
exposed on the PCM iface, and the Playback Mask on the mixer iface.

It's a bit confusing to advise to have two related controls on two
separate ifaces, and it looks like the drivers that currently expose
those controls use any combination of the mixer and PCM ifaces.

Let's try to clarify the situation a bit, and encourage to at least have
the controls on the same iface.

Bug: 239396464
Change-Id: Ie0fb033564972f74154c378c644c581dc4d06dfa
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20210525132354.297468-2-maxime@cerno.tech


Signed-off-by: Sugar Zhang <sugar.zhang@rock-chips.com>
(cherry picked from commit aa789953)