Well, I couldn't leave it alone so I ended up with 
```
        let spi_cell_ref: &'static AtomicCell<ConfiguredSpi> =
            singleton!(: AtomicCell<ConfiguredSpi> = AtomicCell::new(spi)).unwrap();

        let spi_device = AtomicDevice::new(spi_cell_ref, mfrc_cs, NoDelay).expect("exclusive");
```