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/char/i8k.c

@@ -98,9 +98,9 @@ struct smm_regs {
 	unsigned int edi __attribute__ ((packed));
 };
 
-static inline char *i8k_get_dmi_data(int field)
+static inline const char *i8k_get_dmi_data(int field)
 {
-	char *dmi_data = dmi_get_system_info(field);
+	const char *dmi_data = dmi_get_system_info(field);
 
 	return dmi_data && *dmi_data ? dmi_data : "?";
 }