serial: fix TIOCSRS485 locking

The RS485 multipoint addressing support for some reason added a new
ADDRB termios cflag which is (only!) updated from one of the RS485
ioctls.

Make sure to take the termios rw semaphore for the right ioctl (i.e.
set, not get).

Fixes: ae50bb27 ("serial: take termios_rwsem for ->rs485_config() & pass termios as param")
Cc: stable@vger.kernel.org	# 6.0
Cc: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20230412124811.11217-1-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -1549,7 +1549,7 @@ uart_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg)
 		goto out;
 
 	/* rs485_config requires more locking than others */
-	if (cmd == TIOCGRS485)
+	if (cmd == TIOCSRS485)
 		down_write(&tty->termios_rwsem);
 
 	mutex_lock(&port->mutex);
@@ -1592,7 +1592,7 @@ uart_ioctl(struct tty_struct *tty, unsigned int cmd, unsigned long arg)
 	}
 out_up:
 	mutex_unlock(&port->mutex);
-	if (cmd == TIOCGRS485)
+	if (cmd == TIOCSRS485)
 		up_write(&tty->termios_rwsem);
 out:
 	return ret;