[ALSA] intel8x0 - Add support of 8 channel sound

Added the support of 8 channel sound for codecs that are known to work.
So far, only ALC850 is marked as a 8ch-support codec.

This fix is a modified version of the patch on ALSA BTS#2097 by
Martin Ellis:
	https://bugtrack.alsa-project.org/alsa-bug/view.php?id=2097



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