PCI/ASPM: Clarify that pcie_aspm=off means leave ASPM untouched

Previously we claimed "pcie_aspm=off" meant that ASPM would be disabled,
which is wrong.

Correct this to say that with "pcie_aspm=off", Linux doesn't touch any ASPM
configuration at all.  ASPM may have been enabled by firmware, and that
will be left unchanged.  See "aspm_support_enabled".

Link: https://lore.kernel.org/r/20240429191821.691726-1-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: David E. Box <david.e.box@linux.intel.com>