mmc: core: start to break apart mmc_start_areq()

This function is doing to many clever things at the same time under
too many various conditions.

Start to make things clearer by refactoring: break out the
finalization of the previous asynchronous request to its own
function mmc_finalize_areq(). We can get rid of the default
assignment of status and let the call deal with this.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>