ALSA: usb-audio: Add mixer support for Pioneer DJ DJM-250MK2

This patch extends support for DJM-250MK2 and allows mapping
playback and capture channels to available sources.
Configures the card through USB commands.

Signed-off-by: František Kučera <franta-linux@frantovo.cz>
Link: https://lore.kernel.org/r/20200922144206.10472-1-konference@frantovo.cz


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