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.

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