usb: gadget: midi: f_midi_alloc() can be static

drivers/usb/gadget/function/f_midi.c:1072:21: sparse: symbol 'f_midi_alloc' was not declared. Should it be static?
drivers/usb/gadget/legacy/gmidi.c:118:30: sparse: symbol 'fi_midi' was not declared. Should it be static?
drivers/usb/gadget/legacy/gmidi.c:119:21: sparse: symbol 'f_midi' was not declared. Should it be static?

BACKPORT FROM MAINLINE KERNEL

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Change-Id: I7794cd62da8deb408848e86163747d5ce655f33b