ALSA: oxygen: add mute mask for the OXYGEN_PLAY_ROUTING register

The Xonar DG/DGX driver needs this mask to mute unnecessary
channels.
Signed-off-by: Roman Volkov <v1ron@mail.ru>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

sound/pci/oxygen/oxygen_regs.h

@@ -318,6 +318,7 @@
 #define  OXYGEN_PLAY_MUTE23		0x0002
 #define  OXYGEN_PLAY_MUTE45		0x0004
 #define  OXYGEN_PLAY_MUTE67		0x0008
+#define  OXYGEN_PLAY_MUTE_MASK		0x000f
 #define  OXYGEN_PLAY_MULTICH_MASK	0x0010
 #define  OXYGEN_PLAY_MULTICH_I2S_DAC	0x0000
 #define  OXYGEN_PLAY_MULTICH_AC97	0x0010