ALSA: usb-audio: use usb headers rather than define structs locally

Use struct definitions from linux/usb/midi.h rather than locally
define the structs in sound/usb/midi.c.

Signed-off-by: George Harker <george@george-graphics.co.uk>
Link: https://lore.kernel.org/r/20210226212457.24538-1-george@george-graphics.co.uk
Signed-off-by: Takashi Iwai <tiwai@suse.de>