fastboot: rockchip: Fix rk3328 default mmc device

Technically the default mmc device for fastboot would
use eMMC if the board support for it.

Rockchip platform access device 0 for eMMC so, use the
same device number for rk3328.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>