mmc: core: use kobj_to_dev()

Use kobj_to_dev() instead of open-coding it.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Link: https://lore.kernel.org/r/20220425105339.3515368-1-chi.minghao@zte.com.cn


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>