ALSA: usb-audio: Add Xonar U1 resume support

This time it's about Xonar U1: add the proper resume support for
"Digital Playback Switch" element.

Also, the status is moved into kcontrol private_value from
usb_mixer_interface struct field.  One more cut.

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