x86, mrst: Don't blindly access extended config space

Do not blindly access extended configuration space unless we actively
know we're on a Moorestown platform.  The fixed-size BAR capability
lives in the extended configuration space, and thus is not applicable
if the configuration space isn't appropriately sized.

This fixes booting certain VMware configurations with CONFIG_MRST=y.

Moorestown will add a fake PCI-X 266 capability to advertise the
presence of extended configuration space.

Reported-and-tested-by: Petr Vandrovec <petr@vandrovec.name>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Acked-by: Jacob Pan <jacob.jun.pan@intel.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
LKML-Reference: <AANLkTiltKUa3TrKR1M51eGw8FLNoQJSLT0k0_K5X3-OJ@mail.gmail.com>