mmc: jz4740: Use dev_get_platdata

Instead of accessing the platform data pointer directly,
use the dev_get_platdata() helper.

Tested-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.co.uk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>