rockchip: boot_mode: fix fastboot command

The USB controller index must be separated from the type argument,
otherwise the preboot command fails with the error:

	Error: Wrong USB controller index format

Add the missing space to fix fastboot mode here.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>