PCI/AER: Return error if AER is not supported

get_device_error_info() reads error information from registers in the AER
capability.  If we call it for a device that has no AER capability, it
should return an error, but previously it returned success.

Return 0 (error) if the device doesn't have an AER capability.

Signed-off-by: Keith Busch <keith.busch@intel.com>
[bhelgaas: changelog]
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>