mmc: sh_mobile_sdhi: simplify accessing DT data

By using the helper of_device_get_match_data(), we can skip some
checking and make the code simpler.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>