mmc: dw_mmc: Clear the DDR mode for non-DDR

UHS_REG should be cleared for non-DDR mode. But currently there is
no way to clear DDR mode, if it is already set once. This patch adds
clearing DDR mode for non-DDR mode.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>