mmc: core: move some code in mmc_start_areq()

"previous" is a better name for the variable storing the previous
asynchronous request, better than the opaque name "data" atleast.
We see that we assign the return status to the returned variable
on all code paths, so we might as well just do that immediately
after calling 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>