PCI/AER: Use cached AER Capability offset

Replace pci_find_ext_capability(..., PCI_EXT_CAP_ID_ERR) calls with
pci_dev->aer_cap.

pci_dev->aer_cap is initialized in pci_init_capabilities(), which happens
before any of these users of the AER Capability.

Signed-off-by: Frederick Lawler <fred@fredlawl.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>