usb: gadget: uvc: improve sg exit condition

The exit condition to quit iterating over the sg_list, while encoding
the sg entries, has to consider the case that the dma_len of the entry
could be zero. This patch takes this condition to account.
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20220402232744.3622565-4-m.grzeschik@pengutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/uvc_video.c

@@ -155,7 +155,7 @@ uvc_video_encode_isoc_sg(struct usb_request *req, struct uvc_video *video,
 	sg = sg_next(sg);
 
 	for_each_sg(sg, iter, ureq->sgt.nents - 1, i) {
-		if (!len || !buf->sg)
+		if (!len || !buf->sg || !sg_dma_len(buf->sg))
 			break;
 
 		sg_left = sg_dma_len(buf->sg) - buf->offset;