ALSA: hda - Simplify bound-beep mute control for ALC268

The beep mute switch for ALC268 needs to touch two NIDs, and we used
to apply the bind-mixer stuff.  But the use case for ALC268 is fairly
easy to convert to an open-code in a shorter form.  Since this is the
only user of the bind-ctls, we can clean up the common helper codes
later.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>