sound: usb-audio: Xonar U1 digital output support

Add support for the Asus Xonar U1.  This device is mostly class compliant, but
the digital output requires a vendor-specific request.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>