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/ata/pata_ali.c

@@ -40,7 +40,7 @@
  *	Cable special cases
  */
 
-static struct dmi_system_id cable_dmi_table[] = {
+static const struct dmi_system_id cable_dmi_table[] = {
 	{
 		.ident = "HP Pavilion N5430",
 		.matches = {