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/input/misc/wistron_btns.c

@@ -247,7 +247,7 @@ static int have_wifi;
 static int have_bluetooth;
 static int have_leds;
 
-static int __init dmi_matched(struct dmi_system_id *dmi)
+static int __init dmi_matched(const struct dmi_system_id *dmi)
 {
 	const struct key_entry *key;