PCI: Make pci_error_handlers const

Since pci_error_handlers is just a function table make it const.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Linas Vepstas <linasvepstas@gmail.com>