PCI: Constify pci_dev_type structure

Make this const as it not modified in the file referencing it.  It is only
stored in a const field 'type' of a device structure.  Also, add const to
the variable declaration in the header file.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>