[U-Boot] rockchip: rk3399-firefly: update gmac parameter

[Philipp Tomsich]

> Update the tx_deday and rx_delay to match the timing for
> rk3399-firefly board to improve the stability of gmac data
> transfer.
> 
> Signed-off-by: Kever Yang <kever.yang at rock-chips.com>
> ---
> 
>  arch/arm/dts/rk3399-firefly.dts | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 

Reviewed-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>