pcie: Add endpoint capability initialization wrapper

Fix the awkward API of mangling the caller specified PCIe type and
just provide an interface to initialize an endpoint device.  This
will pick either a regular endpoint or integrated endpoint based on
the bus and return pcie_cap_init to doing exactly what is asked.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
