mmc: core: Calculate size from pointer

Calculate the size from pointer instead of
struct to adhere to linux kernel coding style.

Issue reported by checkpatch.

This commit has no functional changes.

Signed-off-by: Riyan Dhiman <riyandhiman14@gmail.com>
Link: https://lore.kernel.org/r/20240901173309.7124-1-riyandhiman14@gmail.com


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