ALSA: usb-audio: Print more information in stream proc files

For more debug and usability information, add the entry showing the
DSD raw states and the channel mapping in each stream proc file.

Link: https://lore.kernel.org/r/20200419212134.14200-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>