ALSA: usb-audio: export snd_usb_feature_unit_ctl

In order to allow quirks functions to hook up to the standard feature
unit op tables, this patch exports a pointer to the struct that is used
internally.

That way, all the code handling the control can be kept private, and
external code can reference the symbol to re-use it.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/mixer.h b/sound/usb/mixer.h
index 459551a..ae1a14d 100644
--- a/sound/usb/mixer.h
+++ b/sound/usb/mixer.h
@@ -24,7 +24,16 @@
 	u8 xonar_u1_status;
 };
 
-#define MAX_CHANNELS	10	/* max logical channels */
+#define MAX_CHANNELS	16	/* max logical channels */
+
+enum {
+	USB_MIXER_BOOLEAN,
+	USB_MIXER_INV_BOOLEAN,
+	USB_MIXER_S8,
+	USB_MIXER_U8,
+	USB_MIXER_S16,
+	USB_MIXER_U16,
+};
 
 struct usb_mixer_elem_info {
 	struct usb_mixer_interface *mixer;