serial: imx: only count 0->1 transitions for RNG

According to tty_ioctl(4) (from man-pages 4.04) the rng member only
counts 0->1 transitions. For the other signals (DSR, CD, CTS) both edges
are supposed to be counted.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -887,7 +887,7 @@ static void imx_mctrl_check(struct imx_port *sport)
 
 	sport->old_status = status;
 
-	if (changed & TIOCM_RI)
+	if (changed & TIOCM_RI && status & TIOCM_RI)
 		sport->port.icount.rng++;
 	if (changed & TIOCM_DSR)
 		sport->port.icount.dsr++;