mmc: core: Factor out mrq preparation from mmc_start_request()

In preparation to reuse the code for CQE support.

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>