serial: imx: Update cached mctrl value when changing RTS

UART core function uart_update_mctrl relies on a cached value of
modem control lines. This was used but not updated by local RTS
control functions within imx.c. These are used for RS485 line
driver enable signalling. Having an out-of-date value in the cached
mctrl can result in the transmitter being enabled when it shouldn't
be.

Fix this by updating the mctrl value before applying it.

Signed-off-by: Ian Jamison <ian.dev@arkver.com>
Origin: id:8195c96e674517b82a6ff7fe914c7ba0f86e702b.1505375165.git.ian.dev@arkver.com
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -334,15 +334,17 @@ static void imx_port_rts_active(struct imx_port *sport, unsigned long *ucr2)
 {
 	*ucr2 &= ~(UCR2_CTSC | UCR2_CTS);
 
-	mctrl_gpio_set(sport->gpios, sport->port.mctrl | TIOCM_RTS);
+	sport->port.mctrl |= TIOCM_RTS;
+	mctrl_gpio_set(sport->gpios, sport->port.mctrl);
 }
 
 static void imx_port_rts_inactive(struct imx_port *sport, unsigned long *ucr2)
 {
 	*ucr2 &= ~UCR2_CTSC;
 	*ucr2 |= UCR2_CTS;
 
-	mctrl_gpio_set(sport->gpios, sport->port.mctrl & ~TIOCM_RTS);
+	sport->port.mctrl &= ~TIOCM_RTS;
+	mctrl_gpio_set(sport->gpios, sport->port.mctrl);
 }
 
 static void imx_port_rts_auto(struct imx_port *sport, unsigned long *ucr2)