PCI: Assign values to 'pci_obff_signal_type' enumeration constants

'pci_obff_signal_type' is passed between drivers and the kernel API.
This patch explicitly assigns values to the enumeration type's constants
which aids in detecting any future changes or additions that would break
the kernel's ABI.

No functional change.

Signed-off-by: Myron Stowe <myron.stowe@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>