fotg210-udc: Don't DMA more than the buffer can take

Before this, it wrote as much as available into the buffer, even if it
didn't fit.

Fixes: b84a8dee ("usb: gadget: add Faraday fotg210_udc driver")
Signed-off-by: Fabian Vogt <fabian@ritter-vogt.de>
Link: https://lore.kernel.org/r/20210324141115.9384-7-fabian@ritter-vogt.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/fotg210-udc.c

@@ -338,8 +338,9 @@ static void fotg210_start_dma(struct fotg210_ep *ep,
 		} else {
 			buffer = req->req.buf + req->req.actual;
 			length = ioread32(ep->fotg210->reg +
-					FOTG210_FIBCR(ep->epnum - 1));
-			length &= FIBCR_BCFX;
+					FOTG210_FIBCR(ep->epnum - 1)) & FIBCR_BCFX;
+			if (length > req->req.length - req->req.actual)
+				length = req->req.length - req->req.actual;
 		}
 	} else {
 		buffer = req->req.buf + req->req.actual;