mips/PCI: removed unused pci_probe configurability

We never assign anything other than PCI_ASSIGN_ALL_BUSSES to pci_probe,
so just remove the indirection.  If configurability is required in the
future, please use the pci_flags/PCI_REASSIGN_ALL_BUS functionality
as is done for powerpc.

CC: Ralf Baechle <ralf@linux-mips.org>
CC: linux-mips@linux-mips.org
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>