mmc: Use snprintf, not sprintf.

Fix an issue found by klockwork. Just paranoia.

Signed-off-by: JiebingLi <jiebing.li@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Chris Ball <cjb@laptop.org>