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/Makefile b/sound/usb/Makefile
index b0e5597..423d829 100644
--- a/sound/usb/Makefile
+++ b/sound/usb/Makefile
@@ -3,12 +3,13 @@
 #
 
 snd-usb-audio-objs := usbaudio.o usbmixer.o
-snd-usb-lib-objs := usbmidi.o
+snd-usbmidi-lib-objs := usbmidi.o
 
 # Toplevel Module Dependency
-obj-$(CONFIG_SND_USB_AUDIO) += snd-usb-audio.o snd-usb-lib.o
-obj-$(CONFIG_SND_USB_UA101) += snd-usb-lib.o
-obj-$(CONFIG_SND_USB_USX2Y) += snd-usb-lib.o
-obj-$(CONFIG_SND_USB_US122L) += snd-usb-lib.o
+obj-$(CONFIG_SND_USB_AUDIO) += snd-usb-audio.o snd-usbmidi-lib.o
+
+obj-$(CONFIG_SND_USB_UA101) += snd-usbmidi-lib.o
+obj-$(CONFIG_SND_USB_USX2Y) += snd-usbmidi-lib.o
+obj-$(CONFIG_SND_USB_US122L) += snd-usbmidi-lib.o
 
 obj-$(CONFIG_SND) += misc/ usx2y/ caiaq/