PCI/AER: Remove error source from AER struct aer_rpc

The AER struct aer_rpc was carrying a copy of the error source simply as a
temperary variable.  Remove that from the structure and use a stack
variable for the purpose.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>