mmc: block: fix logical error to avoid memory leak

If the MMC_DRV_OP_GET_EXT_CSD request completes successfully, then
ext_csd must be freed, but in one case it was not. Fix that.

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