ALSA: hda/realtek - Skip invalid digital out pins

Some BIOS report invalid pins as digital output pins.  The driver checks
the connection but it doesn't do it fully correctly, and it leaves some
undefined value as the audio-out widget, which makes the driver spewing
warnings.  This patch fixes the issue.

Reference: https://bugzilla.novell.com/show_bug.cgi?id=727348

Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>