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 --cc drivers/usb/gadget/Kconfig
Simple merge

diff --cc drivers/usb/gadget/function/Makefile
Simple merge

diff --cc drivers/usb/gadget/function/f_midi.c
Simple merge

diff --cc drivers/usb/gadget/function/u_midi.h
Simple merge

diff --cc drivers/usb/gadget/legacy/gmidi.c
Simple merge