PCI/ASPM: Don't warn if already in common clock mode

Previously we emitted a warning if we tried to configure common clock mode
the link was already configured to common clock mode by the UEFI BIOS.
Bail out silently in that case instead of emitting the warning:

  pci 0004:00:00.0: ASPM: Could not configure common clock

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
[bhelgaas: changelog]
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>