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: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NKeith Busch <keith.busch@intel.com>