xen/arm: if(pfn_valid(pfn)) call native dma_ops

xen/arm: if(pfn_valid(pfn)) call native dma_ops

Remove code duplication in mm32.c by calling the native dma_ops if the
page is a local page (not a foreign page). Use a simple pfn_valid(pfn)
check to figure out if the page is local, exploiting the fact that dom0
is mapped 1:1, therefore pfn_valid always returns false when called on a
foreign mfn.

Suggested-by: Catalin Marinas <[email protected]>
Signed-off-by: Stefano Stabellini <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>