defmt-rtt: available_buffer_size function Ignore available space

```rust
fn available_buffer_size(read_cursor: usize, write_cursor: usize) -> usize {
    if read_cursor > write_cursor {
        read_cursor - write_cursor - 1
    } else if read_cursor == 0 {
        BUF_SIZE - write_cursor - 1
    } else {
        BUF_SIZE - write_cursor
    }
}

```
In the original implementation, if read < write, the available space between 0 and read would be ignored.