PCI: Update code comment on PCI_EXP_LNKCAP_SLS for PCIe r3.0

PCI: Update code comment on PCI_EXP_LNKCAP_SLS for PCIe r3.0

Niklas notes that the code comment on the PCI_EXP_LNKCAP_SLS macro is
outdated as it reflects the meaning of the field prior to PCIe r3.0.
Update it to avoid confusion.

Closes: https://lore.kernel.org/r/70829798889c6d779ca0f6cd3260a765780d1369.camel@kernel.org
Link: https://lore.kernel.org/r/6152bd17cbe0876365d5f4624fc317529f4bbc85.1734376438.git.lukas@wunner.de
Reported-by: Niklas Schnelle <niks@kernel.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>