usb: gadget: udc-core: Race between disconnect/unbind and setup

usb_gadget_remove_driver() runs through a four-step sequence to shut down
the gadget driver.  For the case of a composite gadget + at91 UDC, this
would look like:

    udc->driver->disconnect(udc->gadget);          // composite_disconnect()
    usb_gadget_disconnect(udc->gadget);            // at91_pullup(gadget, 0)
    udc->driver->unbind(udc->gadget);              // composite_unbind()
    usb_gadget_udc_stop(udc->gadget, udc->driver); // at91_stop()

The UDC driver can receive SETUP packets from the host up until the
point when usb_gadget_disconnect() returns.  On rare occasions, the
gadget driver may see this sequence:

    udc->driver->disconnect(udc->gadget);          // composite_disconnect()
    udc->driver->setup(udc->gadget, &ctrl);        // composite_setup()
    udc->driver->unbind(udc->gadget);              // composite_unbind()

Some gadget drivers, such as composite, assume this will never happen
and crash as a result.

The fix is to quiesce the UDC hardware (via usb_gadget_disconnect)
before running the gadget driver through the disconnect/unbind sequence.

Reviewed-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>