drivers/firmware: const-ify DMI API and internals

Three main sets of changes:

1) dmi_get_system_info() return value should have been marked const,
   since callers should not be changing that data.

2) const-ify DMI internals, since DMI firmware tables should,
   whenever possible, be marked const to ensure we never ever write to
   that data area.

3) const-ify DMI API, to enable marking tables const where possible
   in low-level drivers.

And if we're really lucky, this might enable some additional
optimizations on the part of the compiler.

The bulk of the changes are #2 and #3, which are interrelated.  #1 could
have been a separate patch, but it was so small compared to the others,
it was easier to roll it into this changeset.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
diff --git a/drivers/misc/msi-laptop.c b/drivers/misc/msi-laptop.c
index 349be93..83679c7 100644
--- a/drivers/misc/msi-laptop.c
+++ b/drivers/misc/msi-laptop.c
@@ -283,7 +283,7 @@
 
 /* Initialization */
 
-static int dmi_check_cb(struct dmi_system_id *id)
+static int dmi_check_cb(const struct dmi_system_id *id)
 {
         printk("msi-laptop: Identified laptop model '%s'.\n", id->ident);
         return 0;