usb: gadget: f_midi: remove duplicated code

This code is duplicated from f_midi_start_ep(midi, f, midi->out_ep).

Reviewed-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>