Revert "rockchip: firefly: Add "usb start" to auto-start USB device"

This reverts commit a1903c18.

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@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>