sd: limit 'req.cmd' while using as an array index

While processing standard SD commands, the 'req.cmd' value could
lead to OOB read when used as an index into 'sd_cmd_type' or
'sd_cmd_class' arrays. Limit 'req.cmd' value to avoid such an
access.

Reported-by: Qinghao Tang <luodalongde@gmail.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1453315857-1352-1-git-send-email-ppandit@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
