mmc: dw_mmc: fix more const pointer warnings

The patch "dw_mmc: fix multiple drv_data NULL dereferences" has
unfortunately clashed with my "mmc: dw_mmc: constify dw_mci_idmac_ops
in exynos back-end" patch, causing new warnings to appear.

This should hopefully fix the issue for good.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Chris Ball <cjb@laptop.org>