usb: Don't pass negative length values to e1_line_demux_in()

every so often, the USB transfer completes without data (due to
"unlucky" time alignment between E1 and USB frame clock).  Don't
call the demuxer in that case.

Otherwise the user is confused by error messages like

<0001> intf_line.c:467 (I0:L0) IN ERROR: -4

Change-Id: Ia99f97c2cca44d15a83a54cebe884b343ec44f46

src/usb.c

@@ -107,6 +107,8 @@ struct e1_usb_flow {
 static int
 e1_usb_xfer_in(struct e1_usb_flow *flow, uint8_t *buf, int size)
 {
+	if (size == 0)
+		return 0;
 	return e1_line_demux_in(flow->line, buf + 4, size - 4);
 }