mmc: mmc_test: Disable Command Queue while mmc_test is used

Normal read and write commands may not be used while the command queue is
enabled. Disable the Command Queue when mmc_test is probed and re-enable it
when it is removed.

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