PCI: Add PCI_CLASS_SERIAL_USB_DEVICE definition

PCI-SIG has defined Interface FEh for Base Class 0Ch, Sub-Class 03h as "USB
Device (not host controller)".  It is already being used in various USB
device controller drivers for matching, so add PCI_CLASS_SERIAL_USB_DEVICE
and use it.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>