usb: cdns3: gadget: fix possible memory leak

If cdns3_gadget_start is failed, it never frees cdns3_device structure.
Meanwhile, there is no release function for gadget device, it causes
there is no sync with driver core.

To fix this, we add release function for gadget device, and free
cdns3_device structure at there. Meanwhile, With the new UDC core
APIs, we could work with driver core better to handle memory leak
issue.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>