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/hwmon/abituguru.c

@@ -1449,7 +1449,7 @@ static int __init abituguru_init(void)
 	struct resource res = { .flags = IORESOURCE_IO };
 
 #ifdef CONFIG_DMI
-	char *board_vendor = dmi_get_system_info(DMI_BOARD_VENDOR);
+	const char *board_vendor = dmi_get_system_info(DMI_BOARD_VENDOR);
 
 	/* safety check, refuse to load on non Abit motherboards */
 	if (!force && (!board_vendor ||