alpha/PCI: replace pci_probe_only with pci_flags

Some architectures (alpha, mips, powerpc) have an arch-specific
"pci_probe_only" flag.  Others use PCI_PROBE_ONLY in pci_flags for
the same purpose.  This moves alpha to the pci_flags approach so
generic code can use the same test across all architectures.

CC: linux-alpha@vger.kernel.org
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>