mmc: core: Replace with already defined values for readability

SD_ROCR_S18A is already defined and is used to check the rocr value, so
let's replace with already defined values for readability.

Signed-off-by: ChanWoo Lee <cw9316.lee@samsung.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220706004840.24812-1-cw9316.lee@samsung.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>