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>

(Cherry picked from commit 32522a51c7f44a48dc298fce738795dca5c8fcd6)
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>