[ALSA] Fix NM256 hard lock up

Modules: NM256 driver

Treat the nm256 mixer as a write-only device so as to avoid hangs on
initialisation.

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