firmware/dmi_scan: constify strings

Add const to all DMI string pointers where this is possible.  This fixes a
checkpatch warning.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Joe Perches <joe@perches.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>