mmc: card: Avoid null pointer dereference

After the null check on md the code jumped to cmd_done, which then
will dereference md in mmc_blk_put. This patch avoids the possible
null pointer dereference in that case.

Signed-off-by: Philippe De Swert <philippedeswert@gmail.com>
Reviewed-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>