dmi.c: Duplicate returned strings because they are meant to be freed.

Without this patch dmi_shutdown calls free() on read-only strings.

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>

git-svn-id: svn://flashrom.org/flashrom/trunk@1849 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1