pub fn into_async(mut self) -> Spi<'d, Async> {
        self.set_interrupt_handler(self.spi.handler());
        Spi {
            spi: self.spi,
            _mode: PhantomData,
            guard: self.guard,
            pins: self.pins,
        }
    }
Converts the SPI instance into async mode.
esp-hal/src/spi/master.rs