sound: usb: make the USB MIDI module more independent

Remove the dependecy from the USB MIDI code on the snd_usb_audio
structure.  This allows using the USB MIDI module from another driver
without having to pretend to be the generic USB audio driver.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>