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.c b/sound/usb/mixer.c
index ba19bfd..c22fa76 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -86,16 +86,6 @@
 	const struct usbmix_selector_map *selector_map;
 };
 
-enum {
-	USB_MIXER_BOOLEAN,
-	USB_MIXER_INV_BOOLEAN,
-	USB_MIXER_S8,
-	USB_MIXER_U8,
-	USB_MIXER_S16,
-	USB_MIXER_U16,
-};
-
-
 /*E-mu 0202/0404/0204 eXtension Unit(XU) control*/
 enum {
 	USB_XU_CLOCK_RATE 		= 0xe301,
@@ -985,6 +975,9 @@
 	.put = NULL,
 };
 
+/* This symbol is exported in order to allow the mixer quirks to
+ * hook up to the standard feature unit control mechanism */
+struct snd_kcontrol_new *snd_usb_feature_unit_ctl = &usb_feature_unit_ctl;
 
 /*
  * build a feature control