mmc: block: Use local variables in mmc_blk_data_prep()

Use local variables in mmc_blk_data_prep() in preparation for adding CQE
support which doesn't use the output variables.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>