ALSA: virtuoso: fix Essence ST(X) S/PDIF input

On the Xonar Essence ST/STX, the connector J14 has been confirmed to be
a digital input, so enable it in the driver.

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