mmc: core: mmc_blk_rw_cmd_err - remove unused variable

Fix compilation warning:

drivers/mmc/core/block.c:1563:24: warning: variable ‘mq_rq’ set but not
used [-Wunused-but-set-variable]  struct mmc_queue_req *mq_rq;

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>