usb: chipidea: udc: Consolidate the call of disconnect

The udc-core will call gadget's driver->disconnect, so we should avoid
calling gadget's disconnect again at ci_udc_stop in case the gadget's
unbind free some structs which is still used at gadget's disconnect.

Tested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>