ALSA: virtuoso: Xonar DSX support

commit 4492363251235c4499a2d073c5f09121ea23d39d upstream.

This patch adds support for ASUS - Xonar DSX sound cards. Tested on
openSUSE 12.2 with kernel:
Linux 3.4.6-2.10-desktop #1 SMP PREEMPT Thu Jul 26 09:36:26 UTC 2012 (641c197) x86_64 x86_64 x86_64 GNU/Linux
Works:
 - play sounds
 - adjust volume on master channel.
 - mute .

Since Xonar DS uses the same chip, everything that works for DS should
work for DSX as well.

Signed-off-by: Sergiu Giurgiu <sgiurgiu11@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>