ALSA: usb-audio: Rename and export mixer_vol_tlv

Rename mixer_vol_tlv to snd_usb_mixer_vol_tlv and export it to make
it reuseable in mixer_quirks.c.

Signed-off-by: Felix Homann <linuxaudio@showlabor.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/mixer.h b/sound/usb/mixer.h
index 81b2d8a..a7f3d45 100644
--- a/sound/usb/mixer.h
+++ b/sound/usb/mixer.h
@@ -68,4 +68,7 @@
 int snd_usb_mixer_add_control(struct usb_mixer_interface *mixer,
 			      struct snd_kcontrol *kctl);
 
+int snd_usb_mixer_vol_tlv(struct snd_kcontrol *kcontrol, int op_flag,
+			  unsigned int size, unsigned int __user *_tlv);
+
 #endif /* __USBMIXER_H */