PCI: Enable PCIe AER only after checking firmware support

The PCIe port driver currently sets the PCIe AER error reporting bits for
any root or switch port without first checking to see if firmware will grant
control. This patch moves setting these bits to the AER service driver
aer_enable_port routine.  The bits are then set for the root port and any
downstream switch ports after the check for firmware support (aer_osc_setup)
is made. The patch also unsets the bits in a similar fashion when the AER
service driver is unloaded.

Reviewed-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Andrew Patterson <andrew.patterson@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@hobbes.lan>