ALSA: hda - Add digital beep playback switch for STAC/IDT codecs

The digital beep widget may have no mute control, and always enabling
the beep is ofen pretty annoying, especially on laptops.

This patch adds a mixer control "PC Beep Playback Switch" when there
is no mixer amp mute is found, and controls it on software.

Reference: Novell bnc#444572
	https://bugzilla.novell.com/show_bug.cgi?id=444572

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