PCI: Add pci_find_next_ext_capability()

Some extended capabilities, e.g., the vendor-specific capability, can
occur several times.  The existing pci_find_ext_capability() only finds
the first occurrence.  This adds pci_find_next_ext_capability(), which
can iterate through all of them.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>