usb: udc: allow adding and removing the same gadget device

allow usb_del_gadget_udc() and usb add_gadget_udc() to be called
repeatedly on the same gadget->dev structure.

We need to clear the gadget->dev structure so that kobject_init()
doesn't complain about already initialized object.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>