PCI: Use PCI_CLASS_SERIAL_USB instead of bare number

be6646bf ("PCI: Prevent xHCI driver from claiming AMD Nolan USB3 DRD
device") added a quirk to override the PCI class code of the AMD Nolan
device.

Use PCI_CLASS_SERIAL_USB instead of a bare number to improve greppability.
Also add a log message about what we're doing.

No functional change except the new message.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Huang Rui <ray.huang@amd.com>
CC: Jason Chang <jason.chang@amd.com>
CC: Felipe Balbi <balbi@ti.com>