usb: gadget: uvc: stop pump thread on video disable

Since the uvc-video gadget driver is using the v4l2 interface,
the streamon and streamoff can be triggered at any times. To ensure
that the pump worker will be closed as soon the userspace is
calling streamoff we synchronize the state of the gadget ensuring
the pump worker to bail out.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20230911140530.2995138-2-m.grzeschik@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>