PCI/portdrv: Remove unnecessary "pcie_ports=auto" parameter

The "pcie_ports=auto" parameter set pcie_ports_disabled and pcie_ports_auto
to their compiled-in defaults, so specifying the parameter is the same as
not using it at all.

Remove the "pcie_ports=auto" parameter and update the documentation.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>