PCI: Shift PCI_CLASS_NOT_DEFINED consistently with other classes

The PCI class in dev->class is a three-byte value comprising a base class,
sub-class, and interface type.  PCI_CLASS_NOT_DEFINED includes the base
class and sub-class, but not the interface type, so it should be shifted to
make space for the interface.  It happens that PCI_CLASS_NOT_DEFINED is
zero, so it doesn't matter in the end, but we should still use it
consistently with other class definitions.

Treat PCI_CLASS_NOT_DEFINED as a base class/sub-class value that should
appear in bits 8-23 of dev->class.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>