mmc: core: refactor asynchronous request finalization

mmc_wait_for_data_req_done() is called in exactly one place,
and having it spread out is making things hard to oversee.
Factor this function into mmc_finalize_areq().

Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>