mmc: block: Use local var for mqrq_cur

A subsequent patch will remove 'mq->mqrq_cur'. Prepare for that by
assigning it to a local variable.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>