ALSA: hda - Fix uninitialized variable

Fix the following compile warning.  kctl should be NULL-initialized.

  sound/pci/hda/patch_realtek.c: In function ‘alc_build_controls’:
  sound/pci/hda/patch_realtek.c:2550:23: warning: ‘kctl’ may be used uninitialized in this function
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/hda/patch_realtek.c

@@ -2547,7 +2547,7 @@ static struct snd_kcontrol_new alc_beep_mixer[] = {
 static int alc_build_controls(struct hda_codec *codec)
 {
 	struct alc_spec *spec = codec->spec;
-	struct snd_kcontrol *kctl;
+	struct snd_kcontrol *kctl = NULL;
 	struct snd_kcontrol_new *knew;
 	int i, j, err;
 	unsigned int u;