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 --cc sound/usb/Makefile
Simple merge

diff --cc sound/usb/misc/ua101.c
Simple merge

diff --cc sound/usb/usbaudio.c
Simple merge

diff --cc sound/usb/usbaudio.h
Simple merge

diff --cc sound/usb/usbmidi.c
Simple merge

diff --cc sound/usb/usbmidi.h
Simple merge

diff --cc sound/usb/usbmixer.c
Simple merge

diff --cc sound/usb/usbmixer.h
Simple merge

diff --cc sound/usb/usx2y/us122l.c
Simple merge

diff --cc sound/usb/usx2y/usbusx2y.h
Simple merge