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

sound/pci/oxygen/xonar_pcm179x.c

@@ -319,6 +319,7 @@ static void pcm1796_registers_init(struct oxygen *chip)
 		pcm1796_write(chip, i, 20,
 			      data->pcm1796_regs[0][20 - PCM1796_REG_BASE]);
 		pcm1796_write(chip, i, 21, 0);
+		gain_offset = 0;
 	}
 }
 
@@ -615,6 +616,7 @@ static void update_pcm1796_volume(struct oxygen *chip)
 				     + gain_offset);
 		pcm1796_write_cached(chip, i, 17, chip->dac_volume[i * 2 + 1]
 				     + gain_offset);
+		gain_offset = 0;
 	}
 }