usb: gadget: uvc: preserve the address passed to kfree()

__uvcg_fill_strm() called from __uvcg_iter_stream_cls()
might have advanced the "data" even if __uvcg_iter_stream_cls()
returns an error, so use a backup copy as an argument to kfree().

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>