PCI/portdrv: Turn off PCIe services during shutdown

Some of the PCIe services such as AER are being left enabled during
shutdown. This might cause spurious AER errors while SOC is being powered
down.

Clean up the PCIe services gracefully during shutdown to clear these false
positives.

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>