ALSA: snd-usb: add snd_usb_audio-wide mutex

This is needed for new card-wide list operations.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/usbaudio.h b/sound/usb/usbaudio.h
index 3e2b035..a16c21d 100644
--- a/sound/usb/usbaudio.h
+++ b/sound/usb/usbaudio.h
@@ -36,6 +36,7 @@
 	struct snd_card *card;
 	struct usb_interface *pm_intf;
 	u32 usb_id;
+	struct mutex mutex;
 	struct mutex shutdown_mutex;
 	unsigned int shutdown:1;
 	unsigned int probing:1;