[ALSA] hda: Fix 5.1 sound in Dell 6stack ALC888 HDA

This patch fixes 5.1 surround output and headphone detection in the
Dell Inspiron 530 and possibly other Dell systems using the ALC888
codec (mode 6stack-dell).

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>