mmc: dw_mmc: remove the unnecessary IS_ERR() checking for ciu/biu clock

If ciu/biu clock are NULL, clk_disable_unprepare should be just
returned. In clk_disable_unprepare(), already checked whether clk is
error or NULL.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>