usb: udc: core: add device_del() call to error pathway

This patch fixes a bug in the error pathway of
usb_add_gadget_udc_release() in udc-core.c.  If the udc registration
fails, the gadget registration is not fully undone; there's a
put_device(&gadget->dev) call but no device_del().

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