mmc: dw_mmc: fix modular build for exynos back-end

The MODULE_DEVICE_TABLE entry for dw_mci_exynos_match
was incorrectly copied from the platform back-end, which
causes this error when building the driver as a loadable
module:

drivers/mmc/host/dw_mmc-exynos.c: At top level:
drivers/mmc/host/dw_mmc-exynos.c:226:34: error: '__mod_of_device_table' aliased to undefined symbol 'dw_mci_pltfm_match'

This patch fixes the problem by just using the correct
string.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Thomas Abraham <thomas.abraham@linaro.org>
Cc: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>