usb: gadget: zero: put function instances on unbind

If function instances are not put on gadget's unbind, their implementation
module's refcount is nonzero and it is impossible to unload them.

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>