PCI/AER: Take reference on error devices

Error handling may be running in parallel with a hot removal.  Reference
count the device during AER handling so the device can not be freed while
AER wants to reference it.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Sinan Kaya <okaya@kernel.org>