PCI/AER: Move aer_irq() declaration to portdrv.h

The aer_irq() declaration is the only thing needed by aer_inject.c.  Move
it to portdrv.h so we eventually get rid of aerdrv.h completely.  No
functional change intended.

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