From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
XEN_PFN_PHYS is only used in one place in swiotlb-xen making things more
complex than need to be.
Remove the definition of XEN_PFN_PHYS and open code the cast in the one
place where it is needed.
Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>