PCI/AER: Remove duplicate pcie_port_bus_type declaration

pcie_port_bus_type is already declared in portdrv.h, so remove the
unnecessary duplicate declaration in aerdrv.h.  No functional change
intended.

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