usb: gadget: configfs: add a method to unregister the gadget

Add a method to unregister the gadget using its config_item.

There can be functions (e.g. mass storage), which in some circumstances
need the gadget stopped. Add a method of stopping the gadget.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>