switchtec: Use generic PCI Vendor ID and Class Code

Move the Microsemi Switchtec PCI Vendor ID (same as
PCI_VENDOR_ID_PMC_Sierra) to pci_ids.h.   Also, replace Microsemi class
constants with the standard PCI definitions.

Signed-off-by: Doug Meyer <dmeyer@gigaio.com>
[bhelgaas: restore SPDX (I assume it was removed by mistake), remove
device ID definitions]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>