mmc: block: Move DRV OP issue function

We will need to access static functions above the pure block layer
operations in the file, so move the driver operations issue
function down so we can see all non-blocklayer symbols.

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