Function softstart 

async fn softstart(
    driver: &mut Driver<SpiTransport<Spi<'static, Async, Master>, Output<'static>, Output<'static>, 2>, 2>,
    base_red: u8,
    base_green: u8,
    base_blue: u8,
    target_brightness: u8,
) -> Result<(), SpiTransportError<<Spi<'static, Async, Master> as ErrorType>::Error>>

Performs a soft-start brightness ramp on all LEDs at initialization.

Linearly steps brightness from 0 up to target_brightness over SOFTSTART_STEPS increments across SOFTSTART_RAMP_MS milliseconds.

Errors

Returns Err if any bus transaction fails during the ramp.