linux/usb/audio.h: split header

- Split the audio.h file in two to clearly denote the differences
  between the standards.
- Add many more defines to audio-v2.h. Most of them are not currently
  used.
- Replaced a magic value with a proper define

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>