ALSA: usb-audio: rework add_control_to_empty()

This patch renames add_control_to_empty() to snd_usb_mixer_add_control()
and exports it, so the quirks functions can make use of it.

Also, as "struct mixer_build" is private to mixer.c, rewrite the
function to take an argument of type "struct usb_mixer_interface"
instead.

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 b4a2c81..459551a 100644
--- a/sound/usb/mixer.h
+++ b/sound/usb/mixer.h
@@ -55,4 +55,7 @@
 void snd_usb_mixer_inactivate(struct usb_mixer_interface *mixer);
 int snd_usb_mixer_activate(struct usb_mixer_interface *mixer);
 
+int snd_usb_mixer_add_control(struct usb_mixer_interface *mixer,
+			      struct snd_kcontrol *kctl);
+
 #endif /* __USBMIXER_H */