[U-Boot] Revert "rockchip: firefly: Add "usb start" to auto-start USB device"

[Philipp Tomsich]

> This reverts commit a1903c18db13e740e6bedb8955b3272dce5104e1.
> 
> It's really bad idea to add "usb start" in preboot, it will spend
> a lot of time to scan usb bus, and most of people do not need this
> feature.
> 
> Signed-off-by: Kever Yang <kever.yang at rock-chips.com>
> ---
> 
>  include/configs/firefly-rk3288.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 

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