ALSA: hda - add digital beep support for ALC268

Added the digital beep support for ALC268.  It was missing in the
last patches.

However, ALC268 has a strange pin use for widget 0x1d, which could be
used as another purpose.  So, the patch adds a check of the beep control
before creating the hook for input layer.

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