PCI/AER: Reuse existing pcie_port_find_device() interface

The port services driver already provides a method to find the pcie_device
for a service.  Export that function, use it from the aer_inject module,
and remove the duplicate functionality.

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