UPSTREAM: usb: gadget: midi: convert to new interface of f_midi

Use the new f_midi interface so that the old can be removed.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Change-Id: Ia2998f90c72514f06911f553905c058868d9971a