usb-ehci: Use PCI DMA stub functions

This updates the usb-ehci device emulation to use the explicit PCI DMA
wrapper to initialize its scatter/gathjer structure.  This means this
driver should not need further changes when the sglist interface is
extended to support IOMMUs.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
