ALSA: usb-audio: header file cleanups

Rename snd-usb-lib to snd-usbmidi-lib as MIDI functions are the only
thing it actually contains. Introduce a new header file to only declare
these functions.

Introduced usbmixer.h for all functions exported by usbmixer.c.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/usbmixer.h b/sound/usb/usbmixer.h
new file mode 100644
index 0000000..e199e4b
--- /dev/null
+++ b/sound/usb/usbmixer.h
@@ -0,0 +1,11 @@
+#ifndef __USBMIXER_H
+#define __USBMIXER_H
+
+int snd_usb_create_mixer(struct snd_usb_audio *chip, int ctrlif,
+			 int ignore_error);
+void snd_usb_mixer_disconnect(struct list_head *p);
+
+void snd_emuusb_set_samplerate(struct snd_usb_audio *chip,
+			unsigned char samplerate_id);
+
+#endif /* __USBMIXER_H */