Legacy gadgets, before converting them to the new function framework,
used to use the name <something>_do_config() for usb_add_config()'s
callback.

This patch changes the name so that it is easier to follow
the convention.
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>