PCI/ASPM: Allow ASPM on links to PCIe-to-PCI/PCI-X Bridges

7d715a6c ("PCI: add PCI Express ASPM support") added the ability for
Linux to enable ASPM, but for some undocumented reason, it didn't enable
ASPM on links where the downstream component is a PCIe-to-PCI/PCI-X Bridge.

Remove this exclusion so we can enable ASPM on these links.

The Dell OptiPlex 7080 mentioned in the bugzilla has a TI XIO2001
PCIe-to-PCI Bridge.  Enabling ASPM on the link leading to it allows the
Intel SoC to enter deeper Package C-states, which is a significant power
savings.

[bhelgaas: commit log]
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=207571
Link: https://lore.kernel.org/r/20200505173423.26968-1-kai.heng.feng@canonical.com


Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>