ALSA: hda - Fix an "unused variable" compile warning

Forgot to remove an unused variable.
  sound/pci/hda/patch_realtek.c: In function ‘alc882_auto_init_analog_input’:
  sound/pci/hda/patch_realtek.c:7018: warning: unused variable ‘vref’

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