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>
diff --git a/arch/i386/pci/common.c b/arch/i386/pci/common.c
index ebc6f3c..07d5223 100644
--- a/arch/i386/pci/common.c
+++ b/arch/i386/pci/common.c
@@ -123,7 +123,7 @@
  * on the kernel command line (which was parsed earlier).
  */
 
-static int __devinit set_bf_sort(struct dmi_system_id *d)
+static int __devinit set_bf_sort(const struct dmi_system_id *d)
 {
 	if (pci_bf_sort == pci_bf_sort_default) {
 		pci_bf_sort = pci_dmi_bf;
@@ -136,7 +136,7 @@
  * Enable renumbering of PCI bus# ranges to reach all PCI busses (Cardbus)
  */
 #ifdef __i386__
-static int __devinit assign_all_busses(struct dmi_system_id *d)
+static int __devinit assign_all_busses(const struct dmi_system_id *d)
 {
 	pci_probe |= PCI_ASSIGN_ALL_BUSSES;
 	printk(KERN_INFO "%s detected: enabling PCI bus# renumbering"