PCI/switchtec: Add 'generation' variable

Add a generation variable passed through the device ID table and test for
Gen3-specific registers.  This will allow us to add Gen4 and other devices
that extend the programming model.

Link: https://lore.kernel.org/r/20200115035648.2578-3-logang@deltatee.com


Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>