ALSA: usb-audio - Add support for USB X-Fi S51 Pro

USB X-Fi S51 Pro volume and mute from the volume knob on the unit.
Compiled and tested with 2.6.39-rc7-git12

Signed-off-by: Mathieu Bouffard <mbouffard@strangequarks.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>