ALSA: emu10k1: introduce higher-level voice manipulation functions

This adds snd_emu10k1_pcm_init_{voices,extra_voice}() and
snd_emu10k1_playback_{un,}mute_voices() to slightly abstract by voice
function and potential stereo property.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Link: https://lore.kernel.org/r/20230523104612.198884-1-oswald.buddenhagen@gmx.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>