ALSA: usb-audio: Add DSD support for Gustard U16/X26 USB Interface

This patch adds native DSD support for Gustard U16/X26 USB Interface.
Tested using VID and fp->dsd_raw method.

Signed-off-by: Justin Song <flyingecar@gmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/CA+9XP1ipsFn+r3bCBKRinQv-JrJ+EHOGBdZWZoMwxFv0R8Y1MQ@mail.gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>