[ALSA] [snd-intel-hda] enable center/LFE speaker on some laptops

This patch adds LFE mixer controls for laptops with a stac9200 and a mono
speaker pin with amplifier.

Signed-off-by: Richard Fish <bigfish@asmallpond.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>