mmc: block: Fix missing blk_put_request()

Ensure blk_get_request() is paired with blk_put_request().

Fixes: 0493f6fe ("mmc: block: Move boot partition locking into a driver op")
Fixes: 627c3ccf ("mmc: debugfs: Move block debugfs into block module")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Cc: stable@vger.kernel.org # 4.13+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>