ALSA: usb-audio: Declare Pioneer DJM-850 mixer controls
Declare audio capture controls to choose the audio source, and also to
set the capture level (in dB).
See https://github.com/nm2107/Pioneer-DJM-850-driver-reverse-engineering/blob/
172fb9a61055960c88c67b7c416fe5bf3609807b/doc/windows-djm-850-setting-utility/mixer-output-tab/README.md
Signed-off-by: Nicolas MURE <nicolas.mure2019@gmail.com>
Link: https://lore.kernel.org/r/20210301152729.18094-3-nicolas.mure2019@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>