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/misc/sony-laptop.c

@@ -807,7 +807,7 @@ static struct sony_nc_event *sony_nc_events;
 /* Vaio C* --maybe also FE*, N* and AR* ?-- special init sequence
  * for Fn keys
  */
-static int sony_nc_C_enable(struct dmi_system_id *id)
+static int sony_nc_C_enable(const struct dmi_system_id *id)
 {
 	int result = 0;
 
@@ -845,7 +845,7 @@ static struct sony_nc_event sony_C_events[] = {
 };
 
 /* SNC-only model map */
-static struct dmi_system_id sony_nc_ids[] = {
+static const struct dmi_system_id sony_nc_ids[] = {
 		{
 			.ident = "Sony Vaio FE Series",
 			.callback = sony_nc_C_enable,