usb-ohci: Use universal DMA helper functions

usb-ohci: Use universal DMA helper functions

The OHCI device emulation can provide both PCI and SysBus OHCI
implementations.  Because of this, it was not previously converted to
use the PCI DMA helper functions.

This patch converts it to use the new universal DMA helper functions.
In the PCI case, it obtains its DMAContext from pci_dma_context(), in
the SysBus case, it uses NULL - i.e. assumes for now that there will
be no IOMMU translation for a SysBus OHCI.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>