PCI/AER: Move pcie_aer_get_firmware_first() to portdrv.h

Move pcie_aer_get_firmware_first() to portdrv.h, where it can be more
easily shared between AER and DPC.  Then DPC no longer needs to include
aer/aerdrv.h.  No functional change intended.

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