mmc: block: Factor out data preparation

Factor out data preparation into a separate function mmc_blk_data_prep()
which can be re-used for command queuing.

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>