USB: serial: fix TIOCMIWAIT return value

Fix regression introduced by commit 143d9d9 ("USB: serial: add
tiocmiwait subdriver operation") which made the ioctl operation return
ENODEV rather than ENOIOCTLCMD when a subdriver TIOCMIWAIT
implementation is missing.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/usb-serial.c

@@ -408,7 +408,7 @@ static int serial_ioctl(struct tty_struct *tty,
 					unsigned int cmd, unsigned long arg)
 {
 	struct usb_serial_port *port = tty->driver_data;
-	int retval = -ENODEV;
+	int retval = -ENOIOCTLCMD;
 
 	dev_dbg(tty->dev, "%s - cmd 0x%.4x\n", __func__, cmd);
 
@@ -420,8 +420,6 @@ static int serial_ioctl(struct tty_struct *tty,
 	default:
 		if (port->serial->type->ioctl)
 			retval = port->serial->type->ioctl(tty, cmd, arg);
-		else
-			retval = -ENOIOCTLCMD;
 	}
 
 	return retval;