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.mdSigned-off-by: NNicolas MURE <nicolas.mure2019@gmail.com>
Link: https://lore.kernel.org/r/20210301152729.18094-3-nicolas.mure2019@gmail.comSigned-off-by: NTakashi Iwai <tiwai@suse.de>