ALSA: hda - Don't initialize CORB/RIRB for single_cmd mode

So far, CORB/RIRB still remains even if the driver is switched to the
single_cmd mode.  The specification says that this should be disabled,
but I hoped this isn't the case; indeed most devices worked together with
CORB/RIRB.

However, Poulsbo (US15W) seems problematic with this setup, and it
requires to disable CORB/RIRB when single_cmd is used.

Now this patch disables CORB/RIRB initialization when the single_cmd
mode is used.  Also the unsolicited event is disabled because it can't
work without RIRB.

Reported-and-tested-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>