usb: gadget: uvc: giveback vb2 buffer on req complete

On uvc_video_encode_isoc_sg the mapped vb2 buffer is returned
to early. Only after the last usb_request worked with the buffer
it is allowed to give it back to vb2. This patch fixes that.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20220402232744.3622565-3-m.grzeschik@pengutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>