serial: omap: don't disable rs485 if rts gpio is missing

There are rs485 transceivers (e.g. MAX13487E/MAX13488E) which
automatically disable or enable the driver and receiver to keep the bus
in the correct state.
In these cases we don't need a GPIO for flow control.

Fixes: 4a0ac0f55b18 ("OMAP: add RS485 support")
Signed-off-by: Dario Binacchi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/omap-serial.c

···
 			serial_out(up, UART_OMAP_SCR, up->scr);
 			res = (port->rs485.flags & SER_RS485_RTS_AFTER_SEND) ?
 				1 : 0;
-			if (gpiod_get_value(up->rts_gpiod) != res) {
+			if (up->rts_gpiod &&
+			    gpiod_get_value(up->rts_gpiod) != res) {
 				if (port->rs485.delay_rts_after_send > 0)
 					mdelay(
 					port->rs485.delay_rts_after_send);
···
 
 		/* if rts not already enabled */
 		res = (port->rs485.flags & SER_RS485_RTS_ON_SEND) ? 1 : 0;
-		if (gpiod_get_value(up->rts_gpiod) != res) {
+		if (up->rts_gpiod && gpiod_get_value(up->rts_gpiod) != res) {
 			gpiod_set_value(up->rts_gpiod, res);
 			if (port->rs485.delay_rts_before_send > 0)
 				mdelay(port->rs485.delay_rts_before_send);
···
 	/* store new config */
 	port->rs485 = *rs485;
 
-	/*
-	 * Just as a precaution, only allow rs485
-	 * to be enabled if the gpio pin is valid
-	 */
 	if (up->rts_gpiod) {
 		/* enable / disable rts */
 		val = (port->rs485.flags & SER_RS485_ENABLED) ?
 			SER_RS485_RTS_AFTER_SEND : SER_RS485_RTS_ON_SEND;
 		val = (port->rs485.flags & val) ? 1 : 0;
 		gpiod_set_value(up->rts_gpiod, val);
-	} else
-		port->rs485.flags &= ~SER_RS485_ENABLED;
+	}
 
 	/* Enable interrupts */
 	up->ier = mode;