powerpc/PCI: replace pci_probe_only with pci_flags

We already use pci_flags, so this just sets pci_flags directly and removes
the intermediate step of figuring out pci_probe_only, then using it to set
pci_flags.

The PCI core provides a pci_flags definition (currently __weak), so drop
the powerpc definitions in favor of that.

CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>