PCI/AER: Move AER severity defines to aer.h

The function aer_recover_queue() is a public interface and the
severity argument uses #defines that are in the private header
pci/pcie/aer/aerdrv.h.

This patch moves the #defines from pci/pcie/aer/aerdrv.h to
include/linux/aer.h.

[bhelgaas: split "remove 'extern' from declarations" to another patch]
Signed-off-by: Betty Dall <betty.dall@hp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>