[ALSA] ymfpci: fix volume handling of the 44.1 kHz slot

The existing code for handling the 44.1 slot's volume has two problems:
the volume is not affected by the 'Wave Playback Volume' mixer control,
and the BUF441OUTVOL register, which is used to control the per-
substream volume for this slot, uses a different scale than the gain
fields of the other slots.
This patch makes the BUF441OUTVOL register a shadow of the
NATIVEDACOUTVOL register so that the Wave volume is consistent for all
substreams.
As a consequence of this, the per-substream PCM volume control gets no
longer activated for the substream using this slot.  The code for
(de)activating the mixer control is moved from the open/close to the
prepare/trigger_stop callbacks so that it is able to determine the
substream's slot.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>