ALSA: ymfpci: add "Playback" to FM Legacy Volume control

YDSXGR_LEGACYOUTVOL is a Playback Volume control for OPL3 FM Synth.

Signed-off-by: Raymond Yau <superquad.vortex2@gmail.com>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>