is this what you mean then ? : 
```rs
static mut SPI_INSTANCE: StaticCell<Mutex<CriticalSectionRawMutex, &'static mut dyn SpiDevice<u16, Error=Infallible>>> = StaticCell::new();
...
        unsafe {
            SPI_INSTANCE.init(Mutex::new(spi));
        }
...
    extern "C" fn transfer16_function(
        _sensor_id: acc_sensor_id_t,
        buffer: *mut u16,
        buffer_length: usize,
    ) {
        let buffer = unsafe { core::slice::from_raw_parts_mut(buffer, buffer_length) };
        unsafe {
            // Borrow a mutable reference to the SpiDevice
            let spi_ref = SPI_INSTANCE.borrow_mut();

            // Perform the SPI transfer
            let unlock = spi_ref.get_mut().deref_mut();
            unlock.write(buffer).expect("SPI transfer failed");
        }
    }
```