PCI: Constify endpoint pci_epf_type device_type

Make this const as it is only stored in the type field of a device
structure, which is const.  Done using Coccinelle.

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