mmc: block: introduce new_areq and old_areq

Recycling the same variable in an x=x+1 fashion may seem
clever here but it makes the code terse and hard to follow
for humans. Introduce a new_areq and old_areq variable so
we see what is going on.

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