PCI: Fix link speed calculation on retrain failure

In the Linux kernel, the following vulnerability has been resolved:

PCI: Fix link speed calculation on retrain failure

When pcie_failed_link_retrain() fails to retrain, it tries to revert to the
previous link speed. However it calculates that speed from the Link
Control 2 register without masking out non-speed bits first.

PCIE_LNKCTL2_TLS2SPEED() converts such incorrect values to
PCI_SPEED_UNKNOWN (0xff), which in turn causes a WARN splat in
pcie_set_target_speed():

pci 0000:00:01.1: [1022:14ed] type 01 class 0x060400 PCIe Root Port
pci 0000:00:01.1: broken device, retraining non-functional downstream link at 2.5GT/s
pci 0000:00:01.1: retraining failed
WARNING: CPU: 1 PID: 1 at drivers/pci/pcie/bwctrl.c:168 pcie_set_target_speed
RDX: 0000000000000001 RSI: 00000000000000ff RDI: ffff9acd82efa000
pcie_failed_link_retrain
pci_device_add
pci_scan_single_device

Mask out the non-speed bits in PCIE_LNKCTL2_TLS2SPEED() and
PCIE_LNKCAP_SLS2SPEED() so they don't incorrectly return PCI_SPEED_UNKNOWN.

[bhelgaas: commit log, add details from https://lore.kernel.org/r/1c92ef6bcb314ee6977839b46b393282e4f52e74.1750684771.git.lukas@wunner.de]