The USB audio device 0db0:419c based on the Realtek ALC4080 chip exposes
all playback volume controls as "PCM". This is makes distinguishing the
individual functions hard.

The added mapping distinguishes all playback volume controls as their
respective function:
 - Speaker              - for back panel output
 - Frontpanel Headphone - for front panel output
 - IEC958               - for digital output on the back panel

This clarifies the individual volume control functions for users.
Signed-off-by: Johannes Schickel <lordhoto@gmail.com>
Link: https://lore.kernel.org/r/20220115140257.8751-1-lordhoto@gmail.comSigned-off-by: Takashi Iwai <tiwai@suse.de>