[POWERPC] Fix OF pci flags parsing

For PCI devices with only io ports, of_bus_pci_get_flags() will fall
through and still mark the resource as IORESOURCE_MEM.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>