usb: gadget: core: flush gadget workqueue after device removal

usb: gadget: core: flush gadget workqueue after device removal

device_del() can lead to new work being scheduled in gadget->work
workqueue. This is observed, for example, with the dwc3 driver with the
following call stack:
  device_del()
    gadget_unbind_driver()
      usb_gadget_disconnect_locked()
        dwc3_gadget_pullup()
	  dwc3_gadget_soft_disconnect()
	    usb_gadget_set_state()
	      schedule_work(&gadget->work)

Move flush_work() after device_del() to ensure the workqueue is cleaned
up.

Fixes: 5702f75375aa9 ("usb: gadget: udc-core: move sysfs_notify() to a workqueue")
Cc: stable <stable@kernel.org>
Signed-off-by: Roy Luo <royluo@google.com>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Reviewed-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/20250204233642.666991-1-royluo@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>