mmc: block: Fix use-after-free issue for rpmb

The data structure member “rpmb->md” was passed to a call of the function
“mmc_blk_put” after a call of the function “put_device”. Reorder these
function calls to keep the data accesses consistent.

Fixes: 1c87f735 ("mmc: block: Fix bug when removing RPMB chardev ")
Signed-off-by: Peng Hao <richard.peng@oppo.com>
Cc: stable@vger.kernel.org
[Uffe: Fixed up mangled patch and updated commit message]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>