USB: gadget: midi: refactor the device code

Make use of the newly added MIDI function in f_midi.c and strip down
the MIDI gadget code radically. Also use the generic framework function
to avoid code duplication and rename some symbols to bring them in sync
with other code in the gadget framework.

[ balbi@ti.com : fix Section mismatch warnings.
	rebased on top of usb_speed_string() patch to
	avoid conflicts. ]

Signed-off-by: Daniel Mack <zonque@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>