Handle more bogus MCFG entries

Some Asus P4 boards seem to have broken MCFG tables with
only a single entry for busses 0-0.  Special case these
and assume they mean all busses can be accessed.
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>