[ALSA] hda - Fix noise on VT1708 codec

We get quite noisy output on the right channel on VT1708 codec
when 24bit samples are used.  Suppress the 24bit support until any
real fix is found.
	https://bugzilla.novell.com/show_bug.cgi?id=390473

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