mmc: dw_mmc: rockchip: select proper card clock

As you know, biu_clk is used for AMBA AHB/APB interface, ciu_clk is
used for communication between host and card devices. The real bus clock
is ciu, so let's rectify it.

Signed-off-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>