usb: gadget: dbgp: add reset API at usb_gadget_driver

Add reset API at usb_gadget_driver, it calls disconnect handler currently,
but may do different things in future.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>