PCI: Fix 64GT/s effective data rate calculation

Unlike the lower rates, the PCIe 64GT/s Data Rate uses 1b/1b encoding, not
128b/130b (PCIe r6.1 sec 1.2, Table 1-1).  Correct the PCIE_SPEED2MBS_ENC()
calculation to reflect that.

Link: https://lore.kernel.org/r/20240102172701.65501-1-ilpo.jarvinen@linux.intel.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>