PCI: Fix link speed calculation on retrain failure

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.

Fixes: de9a6c8d5dbf ("PCI/bwctrl: Add pcie_set_target_speed() to set PCIe Link Speed")
Reported-by: Andrew <andreasx0@protonmail.com>
Closes: https://lore.kernel.org/r/7iNzXbCGpf8yUMJZBQjLdbjPcXrEJqBxy5-bHfppz0ek-h4_-G93b1KUrm106r2VNF2FV_sSq0nENv4RsRIUGnlYZMlQr2ZD2NyB5sdj5aU=@protonmail.com/
Suggested-by: Maciej W. Rozycki <macro@orcam.me.uk>
Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Jiwei Sun <sunjw10@lenovo.com>
[bhelgaas: commit log, add details from https://lore.kernel.org/r/1c92ef6bcb314ee6977839b46b393282e4f52e74.1750684771.git.lukas@wunner.de]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Cc: stable@vger.kernel.org	# v6.13+
Link: https://patch.msgid.link/20250123055155.22648-2-sjiwei@163.com