ALC299 has no loopback mixer, but the driver still tries to add a beep
control over the mixer NID which leads to the error at accessing it.
This patch fixes it by properly declaring mixer_nid=0 for this codec.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=195775
Fixes: 28f1f9b2 ("ALSA: hda/realtek - Add new codec ID ALC299")
Cc: stable@vger.kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>