ALSA: virtuoso: use headphone gain setting only on front DAC

Do not apply the headphone gain offset to any but the front DAC.  These
DACs would not be used in headphone mode, so this saves a few register
writes.

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