mmc: block: rename mmc_active to areq

The mmc_active member of struct mmc_queue_req has a very
confusing name: this is certainly not always "active", it is
the asynchronous request associated by the mmc_queue_req
but it is not guaranteed to be "active" in any sense, such
as being running on the host.

Simply rename this member to "areq".

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