mmc: mediatek: use data instead of mrq parameter from msdc_{un}prepare_data()

We already have 'mrq->data' before calling these two functions, no
need to find it again via 'mrq->data' internally. Also remove local
data variable accordingly.

Signed-off-by: Yue Hu <huyue2@yulong.com>
Link: https://lore.kernel.org/r/20210517100900.1620-1-zbestahu@gmail.com


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