usb: gadget: export composite's setup & disconnect function

The configfs can't use all of composite's hooks because ->bind() and
->unbind() has to be done a little differently. ->disconnect() and
->setup() on the hand can be recycled.
This patch exports them both so configfs can use them.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>