USB: mos7840: fix port-device leak in error path

The driver set the usb-serial port pointers to NULL on errors in attach,
effectively preventing usb-serial core from decrementing the port ref
counters and releasing the port devices and associated data.

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

drivers/usb/serial/mos7840.c

@@ -2587,7 +2587,6 @@ static int mos7840_startup(struct usb_serial *serial)
 		kfree(mos7840_port->ctrl_buf);
 		usb_free_urb(mos7840_port->control_urb);
 		kfree(mos7840_port);
-		serial->port[i] = NULL;
 	}
 	return status;
 }