mmc: remove unused "ddr" parameter in struct mmc_ios

"mmc: dw_mmc: Fix DDR mode support" removed the last user.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>