PCI: Limit config space size for Netronome NFP6000 family

The NFP6000 has an erratum where reading/writing to PCI config space
addresses above 0x600 can cause the NFP to generate PCIe completion
timeouts.

Limit the NFP6000's config space size to 0x600 bytes.

Signed-off-by: Jason S. McMullan <jason.mcmullan@netronome.com>
[simon: edited changelog]
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>