mmc: bcm2835: Use str_read_write() helper

Remove hard-coded strings by using the str_read_write() helper.

Signed-off-by: Yumeng Fang <fang.yumeng@zte.com.cn>
Link: https://lore.kernel.org/r/20250515204201502Kzmj4I6k5Fv2FjqyEjF6n@zte.com.cn


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