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>