[POWERPC] Ensure we only handle PowerMac PCI bus fixup for memory resources

The fixup code that handles the case for PowerMac's that leave bridge
windows open over an inaccessible region should only be applied to
memory resources (IORESOURCE_MEM).  If not we can get it trying to fixup
IORESOURCE_IO on some systems since the other conditions that are used to
detect the case can easily match for IORESOURCE_IO.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>