mtrr, x86: define MTRR_TYPE_INVALID for mtrr_type_lookup()

mtrr, x86: define MTRR_TYPE_INVALID for mtrr_type_lookup()

mtrr_type_lookup() returns 0xFF when it cannot return a valid MTRR memory
type since MTRRs are disabled.  This patch defines MTRR_TYPE_INVALID to
clarify the meaning of this value, and documents its usage.

There is no functional change in this patch.

Signed-off-by: Toshi Kani <toshi.kani@hp.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Robert Elliott <Elliott@hp.com>
Cc: Paul Bolle <pebolle@tiscali.nl>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>