usb: gadget: f_midi: convert to new function interface with backward compatibility

Converting midi to the new function interface requires converting
the USB midi's function code and its users.

This patch converts the f_midi.c to the new function interface.
The file can now be compiled into a separate usb_f_midi.ko module.

The old function interface is provided by means of a preprocessor
conditional directives. After all users are converted, the old interface
can be removed.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/gadget/function/Makefile b/drivers/usb/gadget/function/Makefile
index 90701aa..576eea5 100644
--- a/drivers/usb/gadget/function/Makefile
+++ b/drivers/usb/gadget/function/Makefile
@@ -38,3 +38,5 @@
 obj-$(CONFIG_USB_F_UAC2)	+= usb_f_uac2.o
 usb_f_uvc-y			:= f_uvc.o uvc_queue.o uvc_v4l2.o uvc_video.o
 obj-$(CONFIG_USB_F_UVC)		+= usb_f_uvc.o
+usb_f_midi-y			:= f_midi.o
+obj-$(CONFIG_USB_F_MIDI)	+= usb_f_midi.o