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>

drivers/pnp/pnpbios/core.c

@@ -500,7 +500,7 @@ static int __init pnpbios_probe_system(void)
 	return 0;
 }
 
-static int __init exploding_pnp_bios(struct dmi_system_id *d)
+static int __init exploding_pnp_bios(const struct dmi_system_id *d)
 {
 	printk(KERN_WARNING "%s detected. Disabling PnPBIOS\n", d->ident);
 	return 0;