mmc: core: rename mmc_start_req() to *areq()

mmc: core: rename mmc_start_req() to *areq()

With the coexisting __mmc_start_request(), mmc_start_request()
and __mmc_start_req() it is a bit confusing that mmc_start_req()
actually does not start a normal request, but an asynchronous
request.

Rename it to mmc_start_areq() to make it explicit what the
function is doing, also fix the kerneldoc for this function
while we're at it.

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