mmc: fix

add backport of: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=7472bab236bdee1173412585591329e718f4d324

fixes: https://github.com/beagleboard/kernel/issues/70

Reported-by: Charles Steinkuehler <charles@steinkuehler.net>
Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
3.8.13-bone29 release

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>