ALSA: hda - Fix unsigned comparison in patch_sigmatel.c

Fix the comparison of unsigned int that causes a compile warning below
by changing to the right signed type:
  patch_sigmatel.c: In function ‘stac92xx_vref_set’:
  patch_sigmatel.c:658: warning: comparison of unsigned expression < 0 is always false

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