sound: virtuoso: enable HDAV S/PDIF input

The Xonar HDAV1.3 has a digital input jack, so enable the corresponding
device.

This is not related to the HDMI stuff, which stays unsupported.

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